This patch adds a third mode for templates: REX inference is requestable
at template instantiation time. This reduces the number of recipes
by removing rex()/nonrex() redundancy for many instructions.
This avoids a lot of dereferences, and InstructionFormat are immutable
once they're created. It removes a lot of code that was keeping the
FormatRegistry around, just in case we needed the format. This is more
in line with the way we create Instructions, and make it easy to
reference InstructionFormats in general.
