Add a branch relaxation pass for #72.

Compute exact EBB header offsets and check that branches are in range.

Not implemented yet: Relax branches that are not in range.

Invoke the relax_branches() pass from the 'test binemit' file tests so
they can verify the proper encoding of branch instructions too.

lib/cretonne/src/lib.rs

@@ -10,6 +10,9 @@ pub use write::write_function;
 /// Version number of the cretonne crate.
 pub const VERSION: &'static str = env!("CARGO_PKG_VERSION");
 
+#[macro_use]
+pub mod dbg;
+
 pub mod binemit;
 pub mod flowgraph;
 pub mod dominator_tree;
@@ -22,9 +25,6 @@ pub mod settings;
 pub mod sparse_map;
 pub mod verifier;
 
-#[macro_use]
-pub mod dbg;
-
 mod abi;
 mod constant_hash;
 mod context;