Code review feedback.

* Remove `once-cell` dependency.
* Remove function address `BTreeMap` from `CompiledModule` in favor of binary
  searching finished functions directly.
* Use `with_capacity` when populating `CompiledModule` finished functions and
  trampolines.

cranelift/entity/src/primary.rs

@@ -148,6 +148,28 @@ where
     pub fn into_boxed_slice(self) -> BoxedSlice<K, V> {
         unsafe { BoxedSlice::<K, V>::from_raw(Box::<[V]>::into_raw(self.elems.into_boxed_slice())) }
     }
+
+    /// Performs a binary search on the values with a key extraction function.
+    ///
+    /// Assumes that the values are sorted by the key extracted by the function.
+    ///
+    /// If the value is found then `Ok(K)` is returned, containing the entity key
+    /// of the matching value.
+    ///
+    /// If there are multiple matches, then any one of the matches could be returned.
+    ///
+    /// If the value is not found then Err(K) is returned, containing the entity key
+    /// where a matching element could be inserted while maintaining sorted order.
+    pub fn binary_search_values_by_key<'a, B, F>(&'a self, b: &B, f: F) -> Result<K, K>
+    where
+        F: FnMut(&'a V) -> B,
+        B: Ord,
+    {
+        self.elems
+            .binary_search_by_key(b, f)
+            .map(|i| K::new(i))
+            .map_err(|i| K::new(i))
+    }
 }
 
 impl<K, V> Default for PrimaryMap<K, V>