using uint64 for number of elements

[jtarchie]

I used Swiss to test out a utility I had written. Initially, it used native maps but employed a lot of memory. I can confirm that the switch to Swiss memory usage (and the graph from the blog) is correct.

I had to make a change, however. I had more items that the uint32 could hold a reference to. I changed the types within the library to uint64. I should have chosen a different data structure, a separate issue.

Would you accept a PR that allows two swiss versions of uint32 and uint64? I was thinking of being through generics.