Yes. Other convenience methods were intentionally left out, but this one is pretty fundamental.

The nonmutating-to-mutating initializer will have a `mutating:` label.

This seems obvious enough that I can extend SE-0138, without a new proposal:
https://github.com/apple/swift-evolution/blob/master/proposals/0138-unsaferawbufferpointer.md

Another idea would be to have an `immutable` computed property on UnsafeMutableBufferPointer. Just because it's fewer characters.

Nonmutating-to-mutating should always require an explicit initialiser, of course, but it's a much less common situation.

We definitely want the initializer with no labels. I think the point of the `immutable` property would be to avoid explicitly calling that initializer, since we don't have implicit conversions on BufferPointers. I think that's another issue.

Implemented as part of SE-0184a