KT-4559: Operator To Function intention action follow-up

This is a follow-up for the intention that Jeremy was working on, continued from:
https://github.com/JetBrains/kotlin/pull/387

Fixes: Most of the problems mentioned by Alexey.
Problem: As for the problem with ```invoke()```, I have done some search but have not been able to find a way of looking up what the existing methods for a user-defined class. Have tried a few ways as can be seen in my commented-out code under ```isApplicableCall()```, but with no success. Would appreciate advice for this problem.

On a side note:
I am getting a java out of memory error in IntelliJ whenever I try to run a test with a ```// WITH_RUNTIME``` directive. Any idea how to fix this?