Add macros for creating null-terminated string literals to windows-sys

[kennykerr]

Much like #1891 for the windows crate, this adds simple s! and w! macros that produce null terminated UTF-8 and UTF-16 string literals. These are slightly simpler since the w! macro in this case does not attempt to also support WinRT APIs and merely produces a UTF-16 pointer. As with their windows crate counterparts, the resulting expressions point to const data that is both folded by the compiler and stored in constant pages.