make sizeof a builtin

[StefanKarpinski]

Since the role of sizeof is to tell you the size of the representation of an object, it seems sensible to make it a builtin since the system itself is in the best position to know this. This is one of the few things that really should not be a generic function. In particular, it would be nice to be able to do things like sizeof((1,2//3)) and get a correct answer.

[JeffBezanson]

I agree, but sizeof has been extended to mean "size of canonical binary representation", in the sense of how many bytes write would write. This is most useful for strings.
So we probably need two functions: a builtin that operates on types and gives the size of the internal representation, and a generic function that operates on values.

[vtjnash]

add Core.sizeof and leave Base.sizeof unaltered?

[JeffBezanson]

Yeah, we might have to do that. It's hard to accept any alternate names for this function. Maybe nbytes?

[StefanKarpinski]

I think I the core vs base distinction is ok and we could have the base version default to calling the core one.