Each language version/extension and target now has a "profile" containing
all of the available builtin function prototypes.  These are written in
GLSL, and come directly out of the GLSL spec (except for expanding genType).

A new builtins/ir/ folder contains the hand-written IR for each builtin,
regardless of what version includes it.  Only those definitions that have
prototypes in the profile will be included.

The autogenerated IR for texture builtins is no longer written to disk,
so there's no longer any confusion as to what's hand-written or
generated.

All scripts are now in python instead of perl.