Add stanford hash/swisstable as hashmap

[rurban]

stanford is optimal for integers (i.e. literals divisible by 256), greg7mdp/parallel-hashmap for strings and structs.

Converting swiss table or greg7mdp/parallel-hashmap from C++/SIMD might be too much work. stanford_hash is similar and good enough for now. EDIT: see LIMachi/swiss-table on github.

Try the o1hash from wyhash as default, but add security measures (collision counting or tree conversion, no sorted_vector yet)

[rurban]

See the hmap branch

[rurban]

Most of the work is needed for a proper/better map/umap API. Either

• go with pairs (a custom struct as strint.hh),
• or seperate the key from the value: TK for key type, T for the value. And have seperate free,copy methods for both.