* Revert "Revert count(where:)"
This reverts commit 779ea19a6a.
Now that SE-0220 has been re-accepted, this adds the `count(where:)`
Sequence method to the standard library.
* add count(where:) and tests
* Revise count(where:) documentation
* Remove errant word in abstract
* add a benchmark for ranges and strings with help from @natecook1000
* update benchmark to use Array instead of Range