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`.

1 files changed, 2 insertions, 2 deletions

diff --git a/components/compositing/webview.rs b/components/compositing/webview.rs
index b8698381520..1263f877012 100644
--- a/components/compositing/webview.rs
+++ b/components/compositing/webview.rs
@@ -4,7 +4,7 @@
 
 use std::collections::HashMap;
 
-use msg::constellation_msg::{PipelineId, WebViewId};
+use base::id::{PipelineId, WebViewId};
 use webrender_api::units::DeviceRect;
 
 #[derive(Debug, Default)]
@@ -113,7 +113,7 @@ impl<WebView> WebViewManager<WebView> {
 mod test {
     use std::num::NonZeroU32;
 
-    use msg::constellation_msg::{
+    use base::id::{
         BrowsingContextId, BrowsingContextIndex, PipelineNamespace, PipelineNamespaceId,
         TopLevelBrowsingContextId,
     };