profile: Integrate the time profiler with the macOS signpost

infrastructure.

With this change, if you supply the `-Z signpost` flag, Instruments.app
can display Servo events overlaid with callstack data. Even better, you
can get call stack profiling for individual Servo events (one layout,
one network request, one style recalculation, etc.)

This adds a dependency on the `signpost` crate. On unsupported OS's,
this crate is an no-op.

components/profile_traits/lib.rs

@@ -14,6 +14,8 @@
 
 extern crate ipc_channel;
 extern crate serde;
+extern crate signpost;
+extern crate util;
 
 pub mod energy;
 pub mod mem;