The size of Int63 is always at least 63 bits. On a 64-bit platform it is just an int
(63-bits), and on a 32-bit platform it is an int64.
Because Int63 has different sizes on 32-bit and 64-bit platforms, there are several
pitfalls to be aware of:
- Int63 will behave differently in the case of overflow.
- marshalling Int63 will not work between 32-bit and 64-bit platforms.
unmarshal will segfault.
- bin_io will work, except that it will raise an overflow exception when you send too
large of an int from a 32-bit to a 64-bit platform. This is couterintuitive because
the 32-bit platform has the larger int size.