bufferlist: add bufferlist code

bufferlist: add bufferlist code

Buffer lists are a means to manage disjoint buffers as one buffer. It's also
possible to put many of those buffers into a list.

The idea is that when support is added to various elements, we will be able to
more efficiently slice and dice buffers, reduce the amount of memcpy and also
reduce data passing overhead.

The implementation is kept simple on purpose, reusing all of the memory
management features we have for miniobjects and buffers.

Access to the bufferlist object is done with an iterator, which allows for
efficient iteration and modification of the list.

See #572285