Different platforms have different sizes for wchar_t. For example:

* Linux: 32-bit (originally intended as UCS-4, rarely used in practice)
* Windows: 16-bit (originally intended as UCS-2, but now probably treated as UTF-16)
* BeOS: 8-bit (strictly intended to be UTF-8)

For KJ purposes, we'll assume wchar_t arrays use the UTF encoding appropriate to their size, whatever that may be on the target platform.

This is mainly being added because the Win32 API uses wchar_t heavily.