Foundational Algorithms prototype

FYI: notwithstanding the recent deferral of rotate from the standard
library, I believe such foundational algorithms are very important for
Swift. I have established

with implementations of rotate and stablePartition.

In preparation for the day when we're ready to include more algorithms
in the library, I'd like to encourage pull requests contributing to this
prototype. We can use this process to sort out implementation details,
naming, documentation, and to make reliable judgements about the impact
on library size and complexity.