Document that bignum_core doesn't handle zero-length arrays

[gilles-peskine-arm]

Arithmetic on the singleton set {0} isn't useful, so there's no point in supporting zero-length numbers in bignum_core. This is different from bignum which manipulates variable-length numbers and a zero-length array, with a null pointer, is a perfectly cromulent representation of 0. Likewise bignum_mod and bignum_mod_raw don't need to work with modulo 1 so they never need to manipulate zero-length limb arrays.

• Document this in bignum_core.h.
• Remove any support for zero-length-with-null-pointer in bignum_core.c.
• See if there's any functions in bignum_mod_raw and bignum_mod where this applies as well.