Add test for bug where Network->new_from_integer stringifies incorrectly

If the integer is a Math::Int128 object but version is 4, it stringifies as an
IPv6 address, which is wrong.
Add tests for Address->new_from_integer handling of uint128 objects with version 4
Always numify (well, stringify) uint128 objects for IPv4 Network/Address objects
Use uint128_to_number instead of stringifying
Changes for stringification bug fix