Make abstract def return type warning an error

[Blacksmoke16]

abstract class A
  abstract def foo : Int
end

class B < A
  def foo : String
    "..."
  end
end

In test.cr:6:13

 6 | def foo : String # Compiles alright
               ^
Warning: this method must return Int, which is the return type of the overridden method A#foo(), or a subtype of it, not String

The above warning will become an error in a future Crystal version.

A total of 1 warnings were found.

https://gitter.im/crystal-lang/crystal?at=5f28242861a83168463531de

oh, the return type check of abstract method. Yeah, it would make sense to have that in 1.0 as errors I think

and we could also remove the skip_abstract_def_check flag. it's been a while

\cc @bcardiff