Avoid unnecessary allocations around buffer storage

[fknorr]

The various get_data and set_data functions on buffer storage objects take and return data in raw_buffer_storage instances. While this is good for memory safety in APIs, it forces unnecessary copies when that data is taken from or written into larger data structures as happens in #94 and #114.

This PR replaces raw_buffer_storage with raw pointers for those APIs. Where ownership is required, the new type-erasing smart pointer type unique_payload_ptr is used which either manages an ad-hoc allocation (like raw_buffer_storage without the metadata) or takes ownership of a unique_frame_ptr, but points to its payload.

[psalz]

LGTM! The only thing I find maybe a bit iffy is that we leak the idea of a "payload" into the buffer manager (solely based on the type name), even though it doesn't really need to know about payloads. Not only that, half of the time the thing doesn't actually point to a payload, making it even weirder.

[fknorr]

As discussed offline, the name unique_payload_ptr is going to stay, but both unique_frame_ptr and unique_payload_ptr now have their own headers to decouple the concept of "owning type-erased blobs" from "networking frames".