fix(computed): destroy helper vm (#277)

upon host component destroy.

avoids a memory leak.

fix #270

Author: LinusBorg

src/apis/computed.ts

@@ -40,6 +40,8 @@ export function computed<T>(
     },
   });
 
+  vm && vm.$on('hook:destroyed', () => computedHost.$destroy());
+
   return createRef<T>({
     get: () => (computedHost as any).$$state,
     set: (v: T) => {