Add 64 bit formats to pack() and unpack()

[lt]

This patch adds 4 new format codes to pack() and unpack()

q   signed long long (always 64 bit, machine byte order)
Q   unsigned long long (always 64 bit, machine byte order)
J   unsigned long long (always 64 bit, big endian byte order)
P   unsigned long long (always 64 bit, little endian byte order)

These modes are only available on 64 bit platforms.

Both q and Q mimic their Perl counterparts, however Perl does not have codes for specifying the endianness. I have chosen J and P to represent the 64 bit equivalents of N and V, as these are both formats in Perl that have no real business in PHP. J is for Perls internal integer format, and P is for pointers to structures.

[rdlowrey]

Ohhh sweet. I've needed this for a long time and haven't ever gotten around to implementing it myself. 👍

[lt]

RFC accepted

[hikari-no-yume]

On 32-bit platforms, the 64-bit codes don't exist at all under this patch. I don't think that's ideal. Could you make it produce an error that makes it obvious why they're missing? Something like Error: 64-bit unpack codes are only available on 64-bit platforms. Otherwise I think users might get confused.

[lt]

Squashed commits for merge - error message concern was addressed