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, 4 insertions, 3 deletions

diff --git a/components/layout/flow.rs b/components/layout/flow.rs
index 29152507c12..8cdaa6d521d 100644
--- a/components/layout/flow.rs
+++ b/components/layout/flow.rs
@@ -31,10 +31,9 @@ use std::sync::atomic::Ordering;
 use std::sync::Arc;
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use bitflags::bitflags;
 use euclid::default::{Point2D, Rect, Size2D, Vector2D};
-use gfx_traits::print_tree::PrintTree;
-use gfx_traits::StackingContextId;
 use log::debug;
 use serde::ser::{SerializeStruct, Serializer};
 use serde::Serialize;
@@ -54,7 +53,9 @@ use webrender_api::units::LayoutTransform;
 use crate::block::{BlockFlow, FormattingContextType};
 use crate::context::LayoutContext;
 use crate::display_list::items::ClippingAndScrolling;
-use crate::display_list::{DisplayListBuildState, StackingContextCollectionState};
+use crate::display_list::{
+    DisplayListBuildState, StackingContextCollectionState, StackingContextId,
+};
 use crate::flex::FlexFlow;
 use crate::floats::{Floats, SpeculatedFloatPlacement};
 use crate::flow_list::{FlowList, FlowListIterator, MutFlowListIterator};