String length returns number of bytes instead of number of characters

[progval]

LambdaS5's implementation of prim("strlen", str) (which is used in the desugared version of str.length) uses Caml's String.length, which counts the number of bytes of the string.

However, String.length is supposed to count the number of characters.
The two values are the same for pure ASCII strings, but not for other unicode characters.

For instance:

$ echo "'é'.length" > /tmp/test.js
$ ./obj/s5.d.byte -desugar /tmp/test.js -env ./envs/es5.env -apply -eval
2.

$ nodejs
> 'é'.length
1