Issue Description:

When a Collection is capable of providing `UnsafeBufferPointer<Element>`, we can provide a significantly faster (and smaller in code size) implementation of `map` and other algorithms on top of that, as we can skip indexing/iterator overhead, bounds checks, improve compilation time, etc., all without requiring total inlinability into user code.

For example, see #21104 which provides 2x speedup to MapReduceString.