Add aligned vs. unaligned benches with two slices where index into the second is read from the first

[hsivonen]

As a follow-up to #1391, it would be relevant to have a benchmark that more closely tests the access pattern that is relevant to CodePointTrie: Having two slices where the index to read from the second slice depends on what is read from the first slice.

The point being checking if unaligned reads pessimize data dependencies in execution.