od: replace lookup tables with simple arithmetic

od: replace lookup tables with simple arithmetic

* src/od.c (FMT_BYTES_ALLOCATED): Use a simpler formula.
Although slightly too generous, the storage wasted is very small
and it pacifies gcc -Wformat-overflow=2.
(bytes_to_oct_digits, bytes_to_signed_dec_digits)
(bytes_to_unsigned_dec_digits, bytes_to_hex_digoits): Remove.
All uses replaced by algorithmic calculations, which are good
enough: they are valid for integers up to 2620 bits (!) and might
be slightly conservative for wider integers.  Remove related
static_asserts, which are no longer needed.