A counted for loop needs a stable limit to run its counter in integer
storage. Array lengths qualified when the loop provably keeps them, but
a string's length did not, so `for (let i = 0; i < s.length; i++)`
kept a double counter and evaluated the length, through the UTF-16
mapping of a non-ASCII string, on every iteration. A string's length
cannot change, so it is now stable whenever the string's binding is
not rebound in the loop or boxed.
Chris Tate
71e118be7b