Break entity references and instruction info out into new modules.

Avoid gathering too much code in repr.rs.

The `entities` module contains entity reference types, and the `instructions`
module contains instruction opcodes and formats.

src/libcretonne/lib.rs

@@ -7,5 +7,7 @@
 
 pub mod types;
 pub mod immediates;
+pub mod entities;
+pub mod instructions;
 pub mod repr;
 pub mod write;