This PR migrates dirs-sys-next from winapi to windows-sys.

Windows-sys is actively maintained, by Microsoft, and has recently
started to be adopted in the ecosystem; mio, parking_lot, wasmtime,
and others have moved to windows-sys.

Migrating dirs-sys-next to windows-sys will help me remove one more of
Wasmtime's transitive dependencies on winapi.

This fixes the following clippy error:

```
error: this public function might dereference a raw pointer but is not marked `unsafe`
   --> dirs-sys/src/lib.rs:141:54
    |
141 |             let result = Shell::SHGetKnownFolderPath(folder_id, 0, 0, &mut path_ptr);
    |                                                      ^^^^^^^^^
    |
    = note: `#[deny(clippy::not_unsafe_ptr_arg_deref)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#not_unsafe_ptr_arg_deref
```