compiler: Support layout(set=,binding=)

compiler: Support layout(set=,binding=)

This change allows shaders to specify set and binding.
The values come from BottomIR as packed 32-bit values:
16 high bits for set, 16 low bits for binding.
We allow them to remain together through the backend, then
split apart during resource map creation.

To facilitate this change, we also switched to using the
resource mapping tracked by the compiler, rather than
forcing bindings to match the descriptor set layout, which
was a short term solution.  This results in greatly reduced
binding table size.