Add recipes for date-time using std. Fixes #787.

[rndomd]

fixes #787

• the tests are passing locally with cargo xtask test all
• commits are squashed into one and rebased to latest master
• PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue on PR merge
  • if issue does not exist consider creating it or remove the clause
• non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
• links to docs.rs have wildcard version https://docs.rs/tar/*/tar/struct.Entry.html
• example has standard error handling
• code identifiers in description are in hyperlinked backticks

[`Entry::unpack`]: https://docs.rs/tar/*/tar/struct.Entry.html#method.unpack

• check if CI is happy with your PR

[AndyGauge]

benchmark example actually is just discarded by LLVM so its super ineffective at showing usefulness

[AndyGauge]

None of the three snippets are rustfmt-formatted.

 awk '/^```rust/{flag=1;next}/^```$/{flag=0}flag'
  src/datetime/duration/benchmark.md > /tmp/snippet.rs
  rustfmt --edition 2021 /tmp/snippet.rs
  cat /tmp/snippet.rs   # paste this back into the ```rust block

Repeat with convert_to_unix.md. or run cargo fmt on each snippet after pasting itinto a scratch .rs file locally

simplify to Duration::from_secs(1) directly as the math here isn't really highly re-usable.

[AndyGauge]

@randomctl I'm happy to make those suggestions, let me know if you need help getting rustfmt.

[rndomd]

@randomctl I'm happy to make those suggestions, let me know if you need help getting rustfmt.

Done! I thought I had run the rustfmt, maybe I lost it when doing additional changes. Sorry! I think now they are well formatted!

[AndyGauge]

🎉 Thank you!

[AndyGauge]

deployed https://rust-lang-nursery.github.io/rust-cookbook/datetime/duration.html#deadline-arithmetic-with-instant