Implement buffer pointer's `MutableCollection` methods as nonmutating

I’m not sure this has been discussed as the main topic of a thread, but it was broached during the light discussion of SE-0237. I suggested giving the UnsafeBufferPointer types a nonmutating interface to MutableCollection, and that was turned down. In the core team’s first evaluation of the proposal, they cited a concern over having too many overloads. It does seem probable that such overloads might not pull their weight.