Add X11 keymap creation function

WIP

Pass --enable-x11 to configure for now.

Can test with:
    ./test/interactive -x
which should behave exactly like your X keyboard, or:
    ./test/x11
which should be print the same keymap as
    xkbcomp $DISPLAY out.xkb
(minus some constructs we do not support.)

TODO:
- Add more asserts.
- Expand tests.
- Document.
- Commit message.

Signed-off-by: Ran Benita <ran234@gmail.com>