Add first-class types for PCSTR and PCWSTR

[kennykerr]

They're finally here! While the Win32 metadata lacks explicit types for PCSTR and PCWSTR, the metadata gives enough clues as to when they would have existed in the original APIs and my metadata crate now reconstructs that type information. As a result, the windows and windows-sys crates can now offer first-class PCSTR and PCWSTR types, alongside PSTR and PWSTR, thus making const-correct Win32 programming far more natural in Rust.

As these types are synthesized and highly specialized, I've moved them into the windows::core and windows_sys::core modules respectively.

Fixes #1496, fixes #1504