Added ability to call method activities by function pointer

[mfateev]

Makes possible the following trick:

type Activities struct {
...
}

func (a *Activities) RetryTimeoutStableErrorActivity() error {
...
}

And inside the workflow:

	var a *Activities
	err := workflow.ExecuteActivity(ctx, a.RetryTimeoutStableErrorActivity).Get(ctx, nil)

This is possible as Go allows nil function receivers.