Add support for serializing 32-bit integers

Add support for serializing 32-bit integers

Some file formats explicitly require this size, which we should
support explicitly. Note that C++ requires that int be at least 16
bits. In practice on current systems it is always 32 bits, but being
explicit might help future portability if any architecture would use
64-bit int.

Also added some support for unsigned 32- and 64-bit integers where
I saw it was missing and might be useful.

Added some test coverage where it already exists.

Refs #1877

Change-Id: I05efd7d4ba07b695fc4ed7c9d9cac8738873a3a5