Core media model for the sheathe packager.
This crate plays the role of Shaka Packager's media/base: the format-agnostic
abstractions every other crate is built on — elementary streams, samples,
timing, and the shared error type. It deliberately knows nothing about MP4,
DASH, or HLS; those live in sheathe-mp4, sheathe-dash, and
sheathe-hls.