Rewrite C ext to public C api

[haukot]

BDIGIT api is an internal ruby C api, and was removed from public api after merge of Bignum and Fixnum into Integer. Using public API's rb_integer_pack which converts a ruby integer to array of C integers(or longs).

It is slower than the previous BDIGIT api, but much faster than ruby version

                                       user     system      total       real
distance3_bdigit                   0.198673   0.000000   0.198673 (0.198672)
distance3_public                   0.373779   0.000000   0.373779 (0.373777)
distance3_ruby                     1.824285   0.000000   1.824285 (1.824315)

Tested on

ruby:2.7.2-slim
ruby:3.3.3-alpine
ruby:2.7.2-alpine
on ubuntu with ruby 3.2.2

cc https://github.com/Nakilon/dhash-vips/issues/21