Move IR modules under repr/.

Use the cretonne::repr module as a common namespace for sub-modules defining the
in-memory representation of Cretonn IL.

cranelift/src/libcretonne/lib.rs

@@ -7,14 +7,7 @@
 
 pub const VERSION: &'static str = env!("CARGO_PKG_VERSION");
 
-pub mod types;
-pub mod condcodes;
-pub mod immediates;
-pub mod entities;
-pub mod instructions;
 pub mod repr;
-pub mod dfg;
-pub mod layout;
 pub mod write;
 pub mod cfg;