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/module/src/module.rs

@@ -6,8 +6,7 @@
 // shared with `DataContext`?
 
 use cretonne_codegen::entity::{EntityRef, PrimaryMap};
-use cretonne_codegen::result::CodegenError;
-use cretonne_codegen::{binemit, ir, Context};
+use cretonne_codegen::{binemit, ir, CodegenError, Context};
 use data_context::DataContext;
 use std::borrow::ToOwned;
 use std::collections::HashMap;