This needs some render testing, but is destined to be part of master.
This also leads to a variety of other simplifications.
- IO are global symbols, so only need one list of linkage nodes (deferred)
- no longer need parse-context-wide 'inEntryPoint' state, entry-point is localized
- several parts of splitting/flattening are now localized
This PR adds:
1. The "u" register class for RW* objects.
2. --shift-image-bindings (== --sib), analogous to --shift-texture-bindings etc.
3. Case insensitive reg classes.
4. Tests for above.
