This allows setting a pre-compiled keymap while ensuring the keymap was created under the same keyboard handle, and more importantly, the same xkbcommon context within the same thread.

This is related to stefanboca/niri#1 which adds per-device keyboard layout. To support this, if the keymap of a keyboard is different from that of the last used one, then we need to set the new keymap. Currently, the only methods to achieve this are set_keymap_from_string and set_xkb_config, which both require compiling the keymap every time on switching. The new get_keymap and set_keymap methods thus allow caching the compiled keymaps, likely in a HashMap, to prevent redundant keymap compilations.