Feature request: more string manipulation functions

[Diggsey]

It would really help if some of these features existed on WideStr and WideCStr:

• Slicing/indexing (more than just RangeFull)
• Mutations (remove, pop, trim, etc.)
• String matching (starts_with, ends_with, contains, find, matches, etc.)
• String splitting
• String replacing
• Iteration (iterate by u16, iterate by (char | malformed utf16), iterate with indices)

It would also be nice to better document the behaviour of to_os_string/from_str on non-windows platforms where there's no canonical 1:1 relationship between an OsString and a WideString.

[starkat99]

Yes, I've been mulling over some of these recently, I definitely want this functionality on WideString.

Currently, the crate doesn't compile on non-windows platforms. Since I originally made it to work with Windows APIs I didn't bother with other platforms. The relationship could definitely be defined better even for Windows platform.