Issue Description:

UTF8View has `withContiguousStorageIfAvailable`, but we would like to add `withContiguousMutableStorageIfAvailable`. The challenge here is that we must validate the contents post-mutation to preserve the validly-encoded UTF-8 contents invariant. However, a `nil` return value communicates that contiguous storage is not available, and even if this wasn't so, we wouldn't want to copy out the entire old contents just in case.

We could consider trapping if the user wrote invalid data in.