This patch adds a benchmark to the Swift benchmark suite based on the
ChaCha20 encryption algorithm.
As Swift evolves it is important that it tackles more and more features
and possible use cases. One of these great use-cases is low-level CPU
intensive code, and cryptographic algorithms are a really important
test-bed.
This benchmark therefore provides a real-world test case for Swift's
optimiser. My ideal outcome here is that Swift should be able to perform
as well at this benchmark as a naive equivalent C implementation.
