Rename the entity_ref module to 'entity'.

This is in preparation for moving a number of data structures into
sub-modules of the 'entity' module:

- EntityList
- EntityMap
- SparseMap

lib/frontend/src/lib.rs

@@ -35,7 +35,7 @@
 //! extern crate cretonne;
 //! extern crate cton_frontend;
 //!
-//! use cretonne::entity_ref::EntityRef;
+//! use cretonne::entity::EntityRef;
 //! use cretonne::ir::{FunctionName, CallConv, Function, Signature, ArgumentType, InstBuilder};
 //! use cretonne::ir::types::*;
 //! use cton_frontend::{ILBuilder, FunctionBuilder};