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/regalloc/coalescing.rs

@@ -18,6 +18,7 @@ use std::cmp::Ordering;
 use std::iter::Peekable;
 use std::mem;
 use isa::{TargetIsa, EncInfo};
+use timing;
 
 /// Dominator forest.
 ///
@@ -282,6 +283,7 @@ impl Coalescing {
         liveness: &mut Liveness,
         virtregs: &mut VirtRegs,
     ) {
+        let _tt = timing::ra_cssa();
         dbg!("Coalescing for:\n{}", func.display(isa));
         let mut context = Context {
             isa,