OFString: Improve API for characters / UTF-32.

With this change, there is a clear separation between characters and
UTF-32 strings now. Characters are just an array of characters in the
native endianess, with no BOM prepended. UTF-32 on the other hand may
have a BOM and can be swapped and is optionally zero-terminated.

This also fixes a few missing UTF-16 init methods in OFMutableString.