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.
This crate contains the metaprogram used by cranelift-codegen. It's not useful on its own.