Handle `if` guards

[KacperFKorban]

avocADO should be able to correctly handle if guards in for comprehensions.
e.g.

for {
  a <- Some(1)
  if a < 2
  b <- Some(2)
} yield b

[KacperFKorban]

Pattern matches in for comprehensions also should be correctly handled. (They depend on withFilter)
e.g.

for {
  (a, b) <- wait.map(_ => 1 -> 2)
  c <- wait.map(_ => a)
} yield (a, b, c)