Issue Description:

This very short, very simple demonstration shows what I mean:

Godbolt.org

Notice that the first function, "copy", gets turned in to an enormous generic specialization which iterates through every position in the slice and copies each byte one-by-one.

The second function, "copy2", uses "withContiguousStorageIfAvailable", and gets turned in to a memcpy.

In my testing, using wCSIA to initialize a destination buffer from slices of a source buffer can be around 8-10% faster than using `initialize(from: )`, depending on the length of the content.

Unfortunately, UMBP does not expose a function which uses wCSIA and otherwise falls-back to naive element-by-element copying. Worse - the current function's signature returns an iterator from the source collection, so the implementation can't be adjusted to make use of it. We need a new function on UMBP.