Work out how to support h265 syntax without duplicating lots of code

[dholroyd]

Lots of h264 syntax, as supported by this crate right now, is the same as h265. i.e. overall structure using Network Abstraction Layer Units.

Work out a good way of sharing code between h264 and h265 parsing, either by making this crate itself more generic, or by breaking common code into a separate crate.

(I don't yet know the differences between h264 and h265 well enough to know exactly where the line between generic and specific is drawn.)

[scottlamb]

I asked about this, but I think now I might not use it even if it existed. The thing is that Chrome and Firefox don't support H.265 which limits its usefulness for me (see here).