This generally means that methods are more efficient, as we
don't need to encode to UTF16 on-the-fly to have correct indices.
This also fix some bugs:
- charCode now properly handle surrogate pairs
- calling lastIndexOf with the empty pattern and an OoB index now
properly returns the string length
Still missing is AVM2's String.match
These types represent an UCS2 string (UTF-16 with unpaired surrogates).
The string is stored either as a sequence of u8s (Bytes) or u16s (Wide);
the type of string is tracked by setting the high bit of the length of
its fat pointer.
Moulins