Add longer k-mers with VarLenKmer and derive macro for KmerSize

[evolp]

Longer k-mers

• new struct VarLenKmer, with all methods required for k-mer functionality are implemented
  • underlying stucture is similar to a DnaString
  • contains an array of generic length, filles with a generic integer
  • this way, k-mers can be implemented as a type with as little unused bits as possible

// this will contain a `[u32, 5]` and know that it should contain 80 bases
type Kmer80 = VarLenKmer<u32, 5, K80>;

• new derive macro for the KmerSize trait, eliminating having to, very repetitively, implement the Kmersize::K() method for each possible value for k.