Safer casts

[Yigal Chripun]

thanks for the added examples and explanations to the proposal.
I want to note two things though:
a) I did suggest cast(!invariant) instead of cast(mutable) but I'm not
sure it's more readable. maybe cast(!const) is a bit better (and
shorter) but the lack of symmetry bothers me.
b) Downcasts - the danger with them is that they can fail if you
downcast to the wrong derived class. that is why I've suggested:
class B : A;
class C : A;
A b = new B;
C c1 = cast!(C)b;
C c2 = cast(C)b;

the first cast will fail and return a null. the exclamation mark should
remind the programmer that this can happen and therefore it's his
responsibility to check the returned value of the cast. This is current
D behavior.

OTOH, the second cast is a "safe" cast and must always work, therefore
the failure to cast will result in an exception being thrown.

another thing: the second cast does not have to fail if the user defined
 his own cast from B to C.

"cast" can be defined by the user, but I'm not sure whether it would be
a good idea to allow the same for "cast!"
what do you think?

--Yigal