Implementation of BIGINT semantic for storing kmers in vizitig DB

Implementation of the BIGINT semantic. Currently, kmers are stored in vizitig's DB as texts, which makes us store redundant data. The idea is (for kmersize <32 & kmersize <64) to store kmers as int (respectively as 1 int or 2 int), making us save a lot of space.

This implementation requires an encoder/decoder approach that needs to be implemented as well.