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/script/dom/htmlscriptelement.rs

@@ -12,6 +12,7 @@ use std::ptr;
 use std::rc::Rc;
 use std::sync::{Arc, Mutex};
 
+use base::id::PipelineId;
 use content_security_policy as csp;
 use dom_struct::dom_struct;
 use encoding_rs::Encoding;
@@ -24,7 +25,6 @@ use js::rust::{
     transform_str_to_source_text, CompileOptionsWrapper, FinishOffThreadStencil, HandleObject,
     Stencil,
 };
-use msg::constellation_msg::PipelineId;
 use net_traits::request::{
     CorsSettings, CredentialsMode, Destination, ParserMetadata, RequestBuilder,
 };