* Add benchmarks for Substring.dropFirst(_:) and dropLast(_:)
These are self-slicing operations, so they should be fast even for
this non-random-access collection.
* Fiddle with the iteration count and length
SubstringComparable had setup overhead of 58 μs (26%).
This was a tricky modification: extracting `substrings` and `comparison` constants out of the run function surprisingly resulted in decreased performance. For some reason this configuration causes significant increase in retain/release traffic. Aliasing the constants in the run function somehow works around this deoptimization.
Also the initial split of the string into 8 substrings takes 44ms!!! (I’m suspecting some king of one-time ICU initialization?)
* Remove case destructuring to _
* Remove some Iterator.Element
* Which idiot wrote this? Oh.
* Switch NibbleSort to just use default impls... shouldn't change perf
