string.pack can't handle longs

[migeyel]

Doing something like ("<I6"):pack(2 ^ 48 - 1) packs into the wrong output because the number gets cast into an int beforehand. Meanwhile, Cobalt casts their inputs into a 64-bit Java long, saturating if the value is too large, so it has the same issue for values larger than 2⁶⁴.

This issue isn't very frequent because most numbers are too small to come up into overflow territory, but since doubles can hold integers all the way over to 2⁵³, it's reasonable to expect to be able to pack these numbers too. Cobalt has some other peculiar choices of rounding, but I think implementing all of them would be too much to ask regardless.