Move non-gfx things out of gfx_traits and create a base crate (#32296)

For a long time, `gfx_traits` has held a lot of things unrelated to graphics
and also unrelated to the `gfx` crate (which is mostly about fonts).
This is a cleanup which does a few things:

1. Move non `gfx` crate things out of `gfx_traits`. This is important in
   order to prevent dependency cycles with a different integration between
   layout, script, and fonts.
2. Rename the `msg` crate to `base`. It didn't really contain anything
   to do with messages and instead mostly holds ids, which are used
   across many different crates in Servo. This new crate will hold the
   *rare* data types that are widely used.

Details:

 - All BackgroundHangMonitor-related things from base to a new
   `background_hang_monitor_api` crate.
 - Moved `TraversalDirection` to `script_traits`
 - Moved `Epoch`-related things from `gfx_traits` to `base`.
 - Moved `PrintTree` to base. This should be widely useful in Servo.
 - Moved `WebrenderApi` from `base` to `webrender_traits` and renamed it
   to `WebRenderFontApi`.

components/net/tests/fetch.rs

@@ -11,6 +11,7 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex, Weak};
 use std::time::{Duration, SystemTime};
 
+use base::id::TEST_PIPELINE_ID;
 use crossbeam_channel::{unbounded, Sender};
 use devtools_traits::{HttpRequest as DevtoolsHttpRequest, HttpResponse as DevtoolsHttpResponse};
 use headers::{
@@ -22,7 +23,6 @@ use http::header::{self, HeaderMap, HeaderName, HeaderValue};
 use http::{Method, StatusCode};
 use hyper::{Body, Request as HyperRequest, Response as HyperResponse};
 use mime::{self, Mime};
-use msg::constellation_msg::TEST_PIPELINE_ID;
 use net::fetch::cors_cache::CorsCache;
 use net::fetch::methods::{self, CancellationListener, FetchContext};
 use net::filemanager_thread::FileManager;