`String.length` not inlined in v10

[cristianoc]

This:

let stringLengthNoSideEffects = String.length("sdkdl")

now produces

var stringLengthNoSideEffects = "sdkdl".length;

while it used to be

var stringLengthNoSideEffects = 5;

[cristianoc]

This is because the parser now sends strings as unicode, and the compiler does not have a length function that matches the one of JS (utf16 code units). Otherwise it could compute it at compile time in string_length inside js_exp_make.ml.

[cristianoc]

The parser might already have such a function and if so one could hook it up to the compiler.