Rename the 'repr' module to 'ir'.

This module and its submodules define the Intermidiate Representation of the
Cretonne IL.

src/libcretonne/lib.rs

@@ -7,7 +7,7 @@
 
 pub const VERSION: &'static str = env!("CARGO_PKG_VERSION");
 
-pub mod repr;
+pub mod ir;
 pub mod write;
 pub mod cfg;