* sort: extract partition from quicksort
Extract the partition logic from quicksort into a macro. This allows to
reuse the partition logic for, e.g., the quickselect algorithm.
* sort: implement quickselect
implement Hoare's selection algorithm (quickselect) on the basis of the
already implemented quicksort. Quickselect allows to find the kth
smallest element in a unordered list with an average time complexity of
O(N) (worst case: O(N^2)).
* add quicksort benchmark
Create a top-level benchmarks folder. Add the benchmark implementation
for the quicksort algorithm.
Benchmarks can then be run in the same way as unit tests from the
root folder with:
c3c compile-benchmarks benchmarks/stdlib/sort
