Allows certain for loops to be transpiled as lua numerical for loops.

for (let i = 0; i < limit; ++i) {}

=>

for i = 0, limit - 1 do
end

• i must be declared with let and not modified inside the loop body
• condition must be one of:
  • i < limit or i <= limit for forward (addition) loops
  • i > limit or i >= limit for reverse (subtraction) loops
• limit must be either:
  • a const or number literal, when condition uses <= or >=
  • a whole number literal, when condition uses < or > (to avoid issues with offsetting)
• the incrementor must be one of:
  • unary increment or decrement (++i, --i, etc...)
  • compound assignment addition or subtraction (i += x or i -= x)
  • assignment to addition or subtraction operation using same variable (i = i + x or i = i - x)

There is also support for a special case:

for (let i = 0, limit = x; i < limit; ++i) {}

=>

for i = 0, x - 1 do
end

Since this is a common paradigm in (oldschool) javscript, it seemed like a good thing to support. Note that it will only be transpiled as a numerical for if the limit variable is not referenced in the the loop body at all.

Also included in this PR:

• for...of loops over arrays now use ipairs instead of a numerical lua for loop
• dis-allowed object destructuring in for...of loops (this silently produced bad code before)
• fixed issues with source mappings of lua for...in statements