See #204

It's too easy to accidentally call functions in `Vec` which may allocate, so
rather than relying on the consumer to use `vec_push` instead of `Vec::push`
and the like, make `TryVec` a thin wrapper around `Vec`, which will return
`Result`s for all the operations which allocate and will use fallible allocation
when the `mp4parse_fallible` feature is enabled.

For now, as a proof of concept, only replace `Vec`s the newly-added AVIF
code. If this works well, expanding to the rest of the code shouldn't be
too much additional effort.

Author: baumanj