vkey 0.2.0

Virtual Keycodes of Windows, Linux, and MacOS
Documentation
# VKey
This crates only contains various types of virtual keycodes, and conversions between them.

This crate is under heavy development, and subject to breaking changes.

- [x] Vk from [winput]https://github.com/gymore-io/winput
- [x] VIRTUAL_KEY from [windows]https://github.com/microsoft/windows-rs
- [ ] enigo::Key from [enigo]https://github.com/enigo-rs/enigo (partially done)
- [x] CGKeyCode from [macos]https://github.com/phracker/MacOSX-SDKs
- [x] Usage from [hut]https://github.com/hidutils/hut
- [ ] winit::KeyCode from [winit]https://github.com/rust-windowing/winit (only definitions)
- [x] xkeysym::KeyCode from [xkeysym]https://github.com/rust-windowing/xkeysym

Following conditions are considered as BUG:
* If any type cannot be accessed from any platform.
* if any type could not be converted to another type.
* if any "mirrored" type is not synced with "underlying" type.

# Limitations
we would also like to add conversion between "mirrored" types and "underlying" types.