Now that we aren't propagating "unsafe" to nested types, remove
unnecessary "unsafe" keywords from the standard library.
(cherry picked from commit fe6856726e)
We have been only including a subset of files and functionality on Embedded Concurrency, let's instead include all the
source files, and have a fine grained opt out on things that don't yet work. Namely, this is still avoiding clocks, task
sleeping and custom executors.
Add a test for AsyncStream and continuations on Embedded Concurrency.
# Motivation
For AsyncStream we created a custom internal Deque implementation to use for the buffering. This implementation was relatively bare-bones compared to the one in swift-collections. Furthermore, it lacked some methods that we need to implement the new `AsyncStream` APIs that support producer backpressure.
# Modification
This PR copies over the Deque implementation of swift-collections and makes it internal and non-inlinable.
# Result
We now have a fully functional Deque.
