**Description**
 - Remove `rustc_hash` dependency
 - Have the fixed length key generator return an array instead of a
   boxed slice

**Motivation**
 - Remove little-used dependency
 - Less memory usage by removing indirection of boxed slice, it shows
   up in the tests

**Testing Done**
`cargo test`

**Description**
Simplify the fixed-length key generator so it returns the expected
number of keys, and no longer spreads the digits over the entire
[0, 255] range.

**Motivation**
This fix was done so that I could generate similarly sized inputs for
the benchmarks.

**Testing Done**
`cargo test`

**Description**
 - Re-organize the criterion benchmarks into a nested structure with a
   single main entrypoint.
 - Convert a couple `debug_assert`s to `assert`s so that there is a
   consistent error message even in release mode

**Motivation**
I wanted to try and share benchmark definitions between criterion and
iai-callgrind, but that didn't really pan out. This change is
in-between and I like that there is only a single benchmark binary
instead of one per-group.

**Testing Done**
`./scripts/full-test.sh nightly`

**Description**
Add `iai-callgrind` benchmarks over the similar set of functions
currently benchmarked by criterion.

**Motivation**
I like the `iai-callgrind` benchmarks because they are very precise
and subsequent runs with no code change return mostly the same
result.

They also run fewer iterations than criterion, and generally take
less time to run. This makes it easier to run them as part of the
developing loop.

**Testing Done**
`./scripts/full-test.sh nightly`