Add Set Operation Methods

[Adamtaranto]

I've added support for set operations on KmerHashTable objects.

I'm very new to Rust, but the test cases I've written are passing. Review with caution!

Summary of changes:

• Simplified count_hash() method so only does one hashmap lookup
• Add get_hash() to query kmer counts using hash key. Returns zero if missing key.
• Add get_hash_array() to lookup many hash keys at once, takes list of keys, returns list of values. Returns zero for missing keys.
• Add .hashes attribute as per sourmash, returns list of all keys in table
• Add set operation methods:

# Perform set operations on the KmerHashTable objects
union_hashes = kmer_table1.union(kmer_table2)
intersection_hashes = kmer_table1.intersection(kmer_table2)
difference_hashes = kmer_table1.difference(kmer_table2)
symmetric_difference_hashes = kmer_table1.symmetric_difference(kmer_table2)

• Add dunder methods to do set operations directly on KmerHashTable objects

  union_hashes = kmer_table1 | kmer_table2          # Union
  intersection_hashes = kmer_table1 & kmer_table2   # Intersection
  difference_hashes = kmer_table1 - kmer_table2     # Difference
  symmetric_difference_hashes = kmer_table1 ^ kmer_table2 # Symmetric Difference

[Adamtaranto]

Feel free to add any code comments, I'm still new to Rust conventions.

What do you think of the function name get_hash_array() - is it confusing that it actually returns a list?

Happy to be a maintainer on PyPi and conda-forge.

Sure, I think this is self-contained enough for a release. Unless you want to wait until wheel issue #15 is fixed.

[Adamtaranto]

Also, should I add a get_array() function for symmetry between the kmer/hash get functions?

i.e. to return counts for an ordered list of kmers.