The following functions are no longer restricted to 16-bit integer values:
String.fromCharCode()
String.prototype.charCodeAt()
repr() will not escape SMP characters, as doing so would require conversion to
surrogate pairs, but will encode these characters as UTF-8. Unicode characters
in the BMP will still be escaped with \uXXXX as before.
JSON.stringify() only escapes control characters, so will represent all non-ASCII
characters as UTF-8.
We do no automatic conversions to/from surrogate pairs. Code that worked with
surrogate pairs should not be affected by these changes.
