Memoized methods of the same instance are mixed

[bencehornak]

If two functions are decorated of the same instance, their memoized values get mixed with the latest version (v9.0.0).

Take for example:

class TestClass {
	@memDecorator()
	fn1() {
		return 1;
	}

	@memDecorator()
	counter2() {
		return 2;
	}
}

const instance = new TestClass();
console.log(instance.fn1()); // prints 1
console.log(instance.fn2()); // also prints 1 !!!

I would expect the second call to return 2, but it returns 1.

The memoized function in my understanding should be connected 1:1 to the combination of instance and propterty, whereas it is currently connected 1:1 to instances (#74).

I guess a little bit more complicated internal data structure is needed instead of the current implementation of decoratorInstanceMap, will open a PR that adds a test and demonstrates a possible solution.