UInt64 can't handle 2**64 - 1

[heftig]

Attempting to construct a UInt64(2 ** 64 - 1) fails with an OverflowError. I think Int64 and UInt64 should be be based on long, not int, as the latter is only guaranteed to fit 32 bits, signed. Even on x86_64 the signedness prevents its use for UInt64.

[cocagne]

That's probably right thing to do. My only concern is whether such a change might break existing applications though I suspect it would not.