anv/entrypoints: Generalize the string map a bit

anv/entrypoints: Generalize the string map a bit

The original string map assumed that the mapping from strings to
entrypoints was a bijection.  This will not be true the moment we
add entrypoint aliasing.  This reworks things to be an arbitrary map
from strings to non-negative signed integers.  The old one also had a
potential bug if we ever had a hash collision because it didn't do the
strcmp inside the lookup loop.  While we're at it, we break things out
into a helpful class.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>