cranelift-wasm: Allow more customization of ref type representations

* Allow different Cranelift IR types to be used for different Wasm reference
  types.

* Do not assume that all Wasm reference types are always a Cranelift IR
  reference type. For example, `funcref`s might not need GC in some
  implementations, and can therefore be represented with a pointer rather than a
  reference type.

cranelift/wasm/src/environ/dummy.rs

@@ -197,6 +197,14 @@ impl<'dummy_environment> DummyFuncEnvironment<'dummy_environment> {
         ));
         sig
     }
+
+    fn reference_type(&self) -> ir::Type {
+        match self.pointer_type() {
+            ir::types::I32 => ir::types::R32,
+            ir::types::I64 => ir::types::R64,
+            _ => panic!("unsupported pointer type"),
+        }
+    }
 }
 
 impl<'dummy_environment> TargetEnvironment for DummyFuncEnvironment<'dummy_environment> {