Make translate_operator() generic on FuncEnvironment.

This makes it clear that this function only uses the FuncEnvironment
trait and not WasmRuntime.

Also make the translate_{grow,current}_memory() methods take &self
instead of &mut self. The &mut was left on there by accident.

lib/wasm/src/runtime/dummy.rs

@@ -89,7 +89,7 @@ impl FuncEnvironment for DummyRuntime {
     }
 
     fn translate_grow_memory(
-        &mut self,
+        &self,
         mut pos: FuncCursor,
         _index: MemoryIndex,
         _heap: ir::Heap,
@@ -99,7 +99,7 @@ impl FuncEnvironment for DummyRuntime {
     }
 
     fn translate_current_memory(
-        &mut self,
+        &self,
         mut pos: FuncCursor,
         _index: MemoryIndex,
         _heap: ir::Heap,