Use an FxHashMap in RegDiversions.

Because it's hot and the number of entries can reach the 1000s, so
linear insertion and search is bad.

This reduces runtime for `sqlite` and `UE4Game-HTML5-Shipping` by 3-4%,
and a couple of other benchmarks (`sqlite`, `godot`, `clang`) by smaller
amounts.

It also increases runtime for `mono` and `tanks` by about 1%; this seems
to be due to incidental changes in which functions are inlined more than
algorithmic changes.

lib/entity/src/map.rs

@@ -97,6 +97,7 @@ where
     }
 
     /// Resize the map to have `n` entries by adding default entries as needed.
+    #[inline]
     pub fn resize(&mut self, n: usize) {
         self.elems.resize(n, self.default.clone());
     }
@@ -125,6 +126,7 @@ where
     K: EntityRef,
     V: Clone,
 {
+    #[inline]
     fn index_mut(&mut self, k: K) -> &mut V {
         let i = k.index();
         if i >= self.elems.len() {