ctx: adapt to the len-aware atom functions

ctx: adapt to the len-aware atom functions

xkb_atom_intern now takes a len parameter. Turns out though that almost
all of our xkb_atom_intern calls are called on string literals, the
length of which we know statically. So we add a macro to micro-optimize
this case.

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