Add tests for the stringology of vizitig

Closes #7 (closed)

Add 9 tests that should fail if the behavior of the stringology functions is modified. This MR also includes a small modification to the is_canonical function. So far it returns a strict equality, ie True or False, but we need a non-strict equality because the kmers can be similar in the case of a revcomp that is the same kmer is the size of the kmers%2==0 (which should not happend, but you never know what users do).