pack_signed_32_big_endian (and other omissions)

[pippijn]

There are a number of functions missing from Binary_packing. E.g. I have to do pack_signed_32 ~byte_order:Big_endian`, which the documentation suggests is up to 3 times slower than a 32 bit _big_endian function. Given that there is a pack_signed_64_big_endian, I see no reason why there shouldn't be one for 32 bit. I'm aware that there is a pack_unsigned_32_int_big_endian (I want unsigned, but for fixed width 32 bit, signed works as well), but I'd like to be portable to 32 bit platforms, where int is signed 31 bit.

[ghost]

Seems reasonable to add these. I'll schedule a patch for the next release.