Export CodegenError and CodegenResult at the top level.

Make cretonne-codegen's `result` module private, and instead just export
`CodegenError` and `CodegenResult` at the top level of the
cretonne-codegen crate. This makes them more consistent with Result and
Error types in other cretonne crates.

lib/codegen/src/lib.rs

@@ -75,7 +75,6 @@ pub mod ir;
 pub mod isa;
 pub mod loop_analysis;
 pub mod print_errors;
-pub mod result;
 pub mod settings;
 pub mod timing;
 pub mod verifier;
@@ -99,6 +98,7 @@ mod predicates;
 mod preopt;
 mod ref_slice;
 mod regalloc;
+mod result;
 mod scoped_hash_map;
 mod simple_gvn;
 mod stack_layout;
@@ -106,6 +106,8 @@ mod topo_order;
 mod unreachable_code;
 mod write;
 
+pub use result::{CodegenError, CodegenResult};
+
 /// This replaces `std` in builds with `core`.
 #[cfg(not(feature = "std"))]
 mod std {