Make keymap thread-safe

[wismill]

(WIP, to open discussion)

The only value that can be modified, once the keymap is compiled, is the reference counter. Use atomics to make it thread-safe.

WARNING: this alone does not make the keymap API thread-safe:

1. It depends on the atom table in the xkb_context for its strings values; this table is not thread-safe.
   However it is “safe” only if no other keymap is compiled using the same context.
2. Functions retrieving text may use the xkb_context internal string buffer, which is not thread safe.
   API not using this buffer should be “safe”.
3. Functions use the xkb_context log function, but modifying it is not thread-safe.
4. State API is not thread-safe.

Fixes #300