Bring in a string type for OS APIs

[alaviss]

On Windows the native encoding is UTF-16. This poses a challenge where typical Nim strings has to converted to UTF-16 before they can be processed.

Right now, the conversion is done using WideCharToMultiByte and its counterpart, which silently drops invalid characters by normalizing them into � (U+FFFD -- Unicode Replacement Character). Furthermore, Windows itself doesn't deal in strict UTF-16, but rather WTF-16 (strings of arbitrary 16-bit codepoints) as a historical artifact.

All of this means that right now, sys operations on Windows have unexpected behaviors when the input string is not well-formed UTF-8 or when the output is not well-formed UTF-16.

To handle this, add a new string type called strings.Native (name subjected to change) that is in OS encoding and provide platform-specific conversion into their native code points.

An open question is to figure out what to do about strings.Without[T]: Whether to have it dealt in strings.Native or to create a new strings.WithoutNative[T].