criterion: cuprate-helper

[hinto-janai]

What

Adds initial Criterion benchmarks for cuprate-helper in benches/criterion/cuprate-helper.

Only the actively used pure functions are tested for now.

Requires #196.

[hinto-janai]

Initial results.

cargo bench -p cuprate-criterion-helper

benchmark	average time per call
integer	1.2848 ns
unsigned	1.2876 ns
combine_low_high_bits_to_u128	846.72 ps
split_u128_into_low_high_bits	647.49 ps
timelock_to_u64	1.6886 ns
u64_to_timelock	1.7356 ns
cmp_float	1.0841 ns
cmp_float_nan	659.60 ps
get_mid	3.9615 ns
median	55.030 ns
tx_fee	6.9573 ns

[hinto-janai]

See #354.