Replace arena allocator

Use slab instead of the deprecated vec_arena, both of which appear to
use the same Vec based storage of an enum that's either the entry or an
index to the next free slot.

sixtyfps_runtime/corelib/Cargo.toml

@@ -34,7 +34,7 @@ scoped-tls-hkt = "0.1"
 static_assertions = "1.1"
 strum = "0.20"
 strum_macros = "0.20"
-vec-arena = "1.0.0"
+slab = "0.4.3"
 pin-weak = "1"
 auto_enums = "0.7"
 stretch = "0.3.2"

sixtyfps_runtime/corelib/graphics.rs

@@ -93,7 +93,7 @@ impl<T> CachedGraphicsData<T> {
 /// The RenderingCache, in combination with CachedGraphicsData, allows backends to store data that's either
 /// intensive to compute or has bad CPU locality. Backends typically keep a RenderingCache instance and use
 /// the item's cached_rendering_data() integer as index in the vec_arena::Arena.
-pub type RenderingCache<T> = vec_arena::Arena<CachedGraphicsData<T>>;
+pub type RenderingCache<T> = slab::Slab<CachedGraphicsData<T>>;
 
 /// FontRequest collects all the developer-configurable properties for fonts, such as family, weight, etc.
 /// It is submitted as a request to the platform font system (i.e. CoreText on macOS) and in exchange the

sixtyfps_runtime/corelib/timers.rs

@@ -162,7 +162,7 @@ struct ActiveTimer {
 /// determining the nearest timeout.
 #[derive(Default)]
 pub struct TimerList {
-    timers: vec_arena::Arena<TimerData>,
+    timers: slab::Slab<TimerData>,
     active_timers: Vec<ActiveTimer>,
     /// If a callback is currently running, this is the id of the currently running callback
     callback_active: Option<usize>,