[stdlib] Remove *Subrange methods from algorithms prototype

Now that slices pin properly, the recursive parts of these algorithms
can operate on collection slices instead of the whole collection with
a `subrange` parameter. Adds tests to make sure we’re successfully
performing the operation without reallocations.

* Fix exclusive access warnings for Algorithms prototype
* Modernize generic constraints and range expressions
* Remove _Subrange methods in Algorithms prototype
* Add slice conformances to MutableCollectionAlgorithms
* Re-indent partitionPoint with 2 spaces
* Add documentation for internal algorithms
* Switch algorithms prototype to use Swift 4.0

Author: natecook1000