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/layout_2020/fragment_tree/base_fragment.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use bitflags::bitflags;
-use gfx_traits::{combine_id_with_fragment_type, FragmentType};
+use script_layout_interface::{combine_id_with_fragment_type, FragmentType};
 use serde::Serialize;
 use style::dom::OpaqueNode;
 use style::selector_parser::PseudoElement;

components/layout_2020/fragment_tree/box_fragment.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use app_units::Au;
-use gfx_traits::print_tree::PrintTree;
+use base::print_tree::PrintTree;
 use serde::Serialize;
 use servo_arc::Arc as ServoArc;
 use style::computed_values::overflow_x::T as ComputedOverflow;

components/layout_2020/fragment_tree/fragment.rs

@@ -5,10 +5,10 @@
 use std::sync::Arc;
 
 use app_units::Au;
+use base::id::{BrowsingContextId, PipelineId};
+use base::print_tree::PrintTree;
 use gfx::font::FontMetrics;
 use gfx::text::glyph::GlyphStore;
-use gfx_traits::print_tree::PrintTree;
-use msg::constellation_msg::{BrowsingContextId, PipelineId};
 use serde::Serialize;
 use servo_arc::Arc as ServoArc;
 use style::properties::ComputedValues;

components/layout_2020/fragment_tree/fragment_tree.rs

@@ -3,9 +3,9 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use app_units::Au;
+use base::print_tree::PrintTree;
 use euclid::default::{Point2D, Rect, Size2D};
 use fxhash::FxHashSet;
-use gfx_traits::print_tree::PrintTree;
 use script_traits::compositor::ScrollSensitivity;
 use serde::Serialize;
 use style::animation::AnimationSetKey;

components/layout_2020/fragment_tree/positioning_fragment.rs

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-use gfx_traits::print_tree::PrintTree;
+use base::print_tree::PrintTree;
 use serde::Serialize;
 use servo_arc::Arc as ServoArc;
 use style::logical_geometry::WritingMode;