list_missing_volumes fails on Windows, because it expects '/' as the path
separator.

Switch over to widestring and W/Ex variants of the unrar_sys functions
everywhere, fixing unicode issues at the cost of using u16/u32
instead of u8 types for strings. Though these are what the underlying
unrar library is also using.

Changed the public Archive API:
- More permissive with the types accepted for archive filename,
  destination and password.
- Return PathBufs rather than Strings when representing filenames.

Also tried to avoid taking ownership of filename inside Archive. But
despite the efforts, all the input strings/paths still end up getting
copied in the conversion to (Wide)CString.

Passwords are still Strings, as the old RARSetPassword interface only
supports char. It will be necessary to switch to using
UCM_NEEDPASSWORDW first. Apparently this is also required for archives
with encrypted headers.

Fixes muja#11, and partially addresses muja#4.

- Add NulError, so that we can combine ffi::NulError and widestring::NulError.
- Check filename and password for nul values on Archive creation.