hoist overflow check from for each num to on usage

This was done to have less branches in the hot path for numbers, thus
has little to no impact on shorter inputs, still a ~5% improvement:

After the change:

    goos: linux
    goarch: amd64
    pkg: github.com/xnacly/go-iso8601-duration
    cpu: AMD Ryzen 7 3700X 8-Core Processor
    BenchmarkDuration/P0D-16                100000000               10.01 ns/op
    BenchmarkDuration/PT15H-16              100000000               11.81 ns/op
    BenchmarkDuration/P1W-16                100000000               10.19 ns/op
    BenchmarkDuration/P15W-16               120588932                9.779 ns/op
    BenchmarkDuration/P1Y15W-16             86265106                13.76 ns/op
    BenchmarkDuration/P15Y-16               122859478                9.910 ns/op
    BenchmarkDuration/P15Y3M-16             77137617                14.32 ns/op
    BenchmarkDuration/P15Y3M41D-16          60263616                20.74 ns/op
    BenchmarkDuration/PT15M-16              96869085                12.30 ns/op
    BenchmarkDuration/PT15M10S-16           63139969                18.33 ns/op
    BenchmarkDuration/P3Y6M4DT12H30M5S-16   29586862                39.14 ns/op
    PASS
    ok      github.com/xnacly/go-iso8601-duration   14.812s

Before the change:

    goos: linux
    goarch: amd64
    pkg: github.com/xnacly/go-iso8601-duration
    cpu: AMD Ryzen 7 3700X 8-Core Processor
    BenchmarkDuration/P0D-16                120725466                9.931 ns/op           0 B/o       0 allocs/op
    BenchmarkDuration/PT15H-16              92520681                11.67 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/P1W-16                100000000               10.15 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/P15W-16               100000000               10.26 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/P1Y15W-16             82810779                14.83 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/P15Y-16               120902158               10.29 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/P15Y3M-16             75393909                15.68 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/P15Y3M41D-16          56214748                22.31 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/PT15M-16              91787005                12.85 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/PT15M10S-16           53683030                19.45 ns/op            0 B/o       0 allocs/op
    BenchmarkDuration/P3Y6M4DT12H30M5S-16   28636116                42.47 ns/op            0 B/op      0 allocs/op
    PASS
    ok      github.com/xnacly/go-iso8601-duration   14.860s

Author: xnacly

duration.go

@@ -101,7 +101,7 @@ func From(s string) (Duration, error) {
 	}
 
 	curState := stateStart
-	var num int64
+	var num uint64
 	var hasNum bool
 
 	for col, b := range []byte(s) {
@@ -122,96 +122,90 @@ func From(s string) (Duration, error) {
 			if b == 'T' {
 				curState = stateT
 			} else if '0' <= b && b <= '9' {
-				num = (num * 10) + int64(b-'0')
+				num = (num * 10) + uint64(b-'0')
 				hasNum = true
 				curState = stateNumber
 			} else {
 				return duration, wrapErr(MissingNumber, col)
 			}
 		case stateNumber:
 			if '0' <= b && b <= '9' {
-				digit := int64(b - '0')
-				if num > (math.MaxInt64-digit)/10 {
-					return duration, DesignatorNumberTooLarge
-				}
-				num = (num * 10) + digit
+				num = (num * 10) + uint64(b-'0')
 				hasNum = true
 				curState = stateNumber
 			} else {
 				if !hasNum {
 					return duration, wrapErr(MissingNumber, col)
 				}
+
+				if num > math.MaxInt64 {
+					return duration, DesignatorNumberTooLarge
+				}
+
 				switch b {
 				case 'Y':
 					if duration.year != 0 {
 						return duration, wrapErr(DuplicateDesignator, col)
 					}
-					duration.year = num
+					duration.year = int64(num)
 				case 'M':
 					if duration.month != 0 {
 						return duration, wrapErr(DuplicateDesignator, col)
 					}
-					duration.month = num
+					duration.month = int64(num)
 				case 'W':
 					if duration.week != 0 {
 						return duration, wrapErr(DuplicateDesignator, col)
 					}
-					duration.week = num
+					duration.week = int64(num)
 				case 'D':
 					if duration.day != 0 {
 						return duration, wrapErr(DuplicateDesignator, col)
 					}
-					duration.day = num
+					duration.day = int64(num)
 				default:
-
 					return duration, wrapErr(UnknownDesignator, col)
 				}
 				num = 0
 				curState = stateDesignator
 			}
 		case stateT, stateTDesignator:
 			if '0' <= b && b <= '9' {
-				digit := int64(b - '0')
-				if num > (math.MaxInt64-digit)/10 {
-					return duration, DesignatorNumberTooLarge
-				}
-				num = (num * 10) + digit
+				num = (num * 10) + uint64(b-'0')
 				hasNum = true
 				curState = stateTNumber
 			} else {
 				return duration, wrapErr(MissingNumber, col)
 			}
 		case stateTNumber:
 			if '0' <= b && b <= '9' {
-				digit := int64(b - '0')
-				if num > (math.MaxInt64-digit)/10 {
-					return duration, DesignatorNumberTooLarge
-				}
-				num = (num * 10) + digit
+				num = (num * 10) + uint64(b-'0')
 				hasNum = true
 				curState = stateTNumber
 			} else {
 				if !hasNum {
 					return duration, wrapErr(MissingNumber, col)
 				}
+				if num > math.MaxInt64 {
+					return duration, DesignatorNumberTooLarge
+				}
 				switch b {
 				case 'H':
 					if duration.hour != 0 {
 						return duration, wrapErr(DuplicateDesignator, col)
 					}
-					duration.hour = num
+					duration.hour = int64(num)
 				case 'M':
 					if duration.minute != 0 {
 						return duration, wrapErr(DuplicateDesignator, col)
 					}
-					duration.minute = num
+					duration.minute = int64(num)
 				case 'S':
 					if duration.second != 0 {
 						return duration, wrapErr(DuplicateDesignator, col)
 					}
-					duration.second = num
+					duration.second = int64(num)
 				default:
-
 					return duration, wrapErr(UnknownDesignator, col)
 				}
 				num = 0