Clean up various things for Keyboard v2

[notgull]

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

Various cleanups for Keyboard V2 (#2789):

• Use the build constants for the platforms.
• Make sure memmap2 is at the latest version and is actually imported.
• Run rustfmt.
• Add documentation for KeyCodeExtScancode.

[kchibisov]

The wayland backend hasn't been updated yet, so I'm not sure wrt that PR yet. Will keep it around for a while.

[notgull]

The wayland backend hasn't been updated yet, so I'm not sure wrt that PR yet. Will keep it around for a while.

I see. Would you like me to take a crack at updating it for Wayland?

[kchibisov]

I'll do that given that it's simple, but I just don't have time at the moment. You can update x11's xkb handling and I just use it, given both use common infra.

[notgull]

I removed the changes to the Wayland from this PR.