Validate enums have a sensible versions and are visible

Add version field for each eumerant.

For capabilities and instructions introduced by an extension (its
first version is "None"):
- the capability should be guarded by an extension
- the instruction should be guarded by a capability.

Other enums are presumed guarded transitiviely by use as an operand
to an instruction or another operand.

Fixes: #278, #368