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/binemit/relaxation.rs

@@ -32,7 +32,7 @@ use cursor::{Cursor, FuncCursor};
 use ir::{Function, InstructionData, Opcode};
 use isa::{EncInfo, TargetIsa};
 use iterators::IteratorExtras;
-use result::CodegenResult;
+use CodegenResult;
 
 /// Relax branches and compute the final layout of EBB headers in `func`.
 ///