split into modules
fix style and imports
Add robin_hood. Optimize and test insert_or_update_with

undesirable inlining:
test hashmap::bench::find_existing    ... bench:     51190 ns/iter (+/- 1524)
test hashmap::bench::find_nonexisting ... bench:     17366 ns/iter (+/- 103)
test hashmap::bench::find_pop_insert  ... bench:       200 ns/iter (+/- 28)
test hashmap::bench::hashmap_as_queue ... bench:       140 ns/iter (+/- 5)
test hashmap::bench::insert           ... bench:       223 ns/iter (+/- 77)
test hashmap::bench::new_drop         ... bench:       123 ns/iter (+/- 1)
test hashmap::bench::new_insert_drop  ... bench:       235 ns/iter (+/- 16)
started documenting
fix warnings