compat: shadow numeric for control variables on 5.4+

[JJH090501]

Closes #1164. Option 1, as decided in the issue: mirror the forin shadow
rather than making numeric control variables const in all targets.

Problem

lua_compat.adjust_code only had a forin case, so plain (non-macro) code that
writes to a numeric for variable type checks and then generates invalid Lua:

local t: {string} = { "a", "b", "c" }

for i = 1, #t do
   i = i + 0
   print(t[i])
end

$ lua5.5 ./tl run fornum.tl
Internal Compiler Error: Teal generator produced invalid Lua. Please report a bug at https://github.com/teal-language/tl

fornum.tl:3: attempt to assign to const variable 'i'

Change

• teal/ast.tl — fornum_modifies_control_var on Node.
• teal/check/visitors.tl — the fornum visitor sets it from
  has_been_written_to, the same way forin does, so writes from nested
  functions are covered too.
• teal/gen/lua_compat.tl — a fornum case. The shadow construction that was
  inlined in the forin case is factored out into shadow_control_var() and
  shared; the new case is six lines.

The shadow is a no-op in every target, since writing to a numeric for variable
has never affected the iteration:

$ lua5.1 -e 'for i=1,3 do i = i + 100; io.write(i," ") end'
101 102 103

Tests

spec/lang/code_gen/fornum_spec.lua, deliberately parallel to forin_spec.lua:
no shadow on 5.3, a shadow on 5.4 when the body assigns, none when it does not,
a loop with an explicit step, an assignment made from a nested function, and
nested loops where only the inner one is written to.

make selfbuild clean, busted --suppress-pending spec/ 1932 → 1938. The
reproduction above was verified end to end on Lua 5.5.1.

[hishamhm]

@JJH090501 Thank you! Rebased and merged!