Add withFilter to FunctorFilter.Ops

[ikeyan]

So for-comprehension with if clauses compiles.

for {
  n <- Try(Seq(1, 2, 3)).nested if n % 2 == 0
} yield n

[BartekBH]

I would like to do this if someone can confirm that it is needed.

EDIT: I took a look at it and added a simple implementation of withFilter based on the standard filter. I added standard function in FunctorFilter trait with comment, and target version in Ops but it is maybe redundand. Additionally, I added a basic test - 'withFilter filter consistency' law.
Now the if statement in the for-comprehension works. However, I'm not sure if this is sufficient; I might be missing something, and it may not be that straightforward.

I don't want to clutter the repository with unnecessary PRs, so if someone could review this comment and approve the approach, I can push my code.