Add a compilation pass timing facility.

Individual compilation passes call the corresponding timing::*()
function and hold on to their timing token while they run. This causes
nested per-pass timing information to be recorded in thread-local
storage.

The --time-passes command line option prints a pass timing report to
stdout.

lib/cretonne/src/lib.rs

@@ -28,6 +28,7 @@ pub mod packed_option;
 pub mod regalloc;
 pub mod result;
 pub mod settings;
+pub mod timing;
 pub mod verifier;
 
 mod abi;