Speed up nested #simple_fields_for usage by a considerable amount

Exceptions are slow due to generating backtraces - and then when other
debugging gems (such as did_you mean, which are included with
Ruby and Rails these days) get involved it resulted in excessive Object
allocation.

lib/simple_form/form_builder.rb

@@ -672,12 +672,7 @@ def mapping_override(klass)
 
     def attempt_mapping(mapping, at)
       return if SimpleForm.inputs_discovery == false && at == Object
-
-      begin
-        at.const_get(mapping)
-      rescue NameError => e
-        raise if e.message !~ /#{mapping}$/
-      end
+      return at.const_get(mapping) if at.const_defined?(mapping)
     end
 
     def attempt_mapping_with_custom_namespace(input_name)