Benchmarks:

```
Without bounds checking

test hashmap::bench::find_existing                              ... bench:        31 ns/iter (+/- 4)
test hashmap::bench::find_nonexisting                           ... bench:        31 ns/iter (+/- 1)
test hashmap::bench::find_pop_insert                            ... bench:       183 ns/iter (+/- 8)
test hashmap::bench::hashmap_as_queue                           ... bench:       120 ns/iter (+/- 3)
test hashmap::bench::insert                                     ... bench:       166 ns/iter (+/- 8)

With bounds checking

test hashmap::bench::find_existing                              ... bench:        37 ns/iter (+/- 4)
test hashmap::bench::find_nonexisting                           ... bench:        36 ns/iter (+/- 7)
test hashmap::bench::find_pop_insert                            ... bench:       221 ns/iter (+/- 16)
test hashmap::bench::hashmap_as_queue                           ... bench:       130 ns/iter (+/- 13)
test hashmap::bench::insert                                     ... bench:       176 ns/iter (+/- 13)
```

I consider that significant. If you disagree, I can re-add bounds checking.