Change inference cache to use a Threadsafe::Cache

* The ThreadSafe::Cache is optimized towards performance while
  giving up some compatibility with the hash interface. It is better
  suited for a cache than ThreadSafe::Hash.

lib/axiom/types.rb

@@ -24,7 +24,7 @@ module Types
     Contradiction = ->(_value) { true }.freeze
 
     # Cache the type inference lookup by object
-    @inference_cache = ThreadSafe::Hash.new do |cache, object|
+    @inference_cache = ThreadSafe::Cache.new do |cache, object|
       type = nil
       Type.descendants.detect do |descendant|
         type = descendant.infer(object)