Check abstract def implementations with splats, default values and keyword arguments

[waj]

Now methods with default arguments are checked that are implemented with the exact same value. In the future this restriction could be relaxed, by making the abstract def to implicitly define an overload with less arguments, just like other methods.

Methods with splats can be implemented exactly as the abstract method defines, but also cases like these are supported:

abstract def foo(*args)
def foo(a = 1, b = 2, *args)

abstract def foo(a : T1, b : T2, *args : T3)
def foo(*args : T1 | T2 | T3)

Keywords arguments are allowed in any order. Extra keyword arguments are allowed if they have a default value.