xkbcomp: Use keycode name LUT for xkb_keycodes
Replace linear search of keycode names with O(1).
Given that:
- the number of atoms is usually < 1k;
- the keycode section usually appears first;
then the first atoms will be mostly the keycodes and their aliases,
so we can achieve O(1) lookup of the key names by using a simple
atom → keycode array.
The LUT will be used also to process `xkb_symbols` for further speedup.
On my setup I get a 1.039x speedup at the costs of less than 1%
additional allocations.