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.
A rebuild after touching components/profile/mem.rs now takes 48 seconds (and
only rebuilds `profile` and `servo`) which is much lower than it used to be.
In comparison, a rebuild after touching components/profile_traits/mem.rs takes
294 seconds and rebuilds many more crates.
This change also removes some unnecessary crate dependencies in `net` and
`net_traits`.
