Opening up structs/traits/fns to make imdl useable as a library

Hey Casey!

I'm writing something torrent-related, and it'd be great to use `intermodal` in tests to verify my implementation. However, currently everything is `pub(crate)`, which makes shelling out to `imdl` the only option. Would you be open to making things just `pub`?

I understand that downstream code directly depending on innards of `imdl` would invite compatibility complaints. However, maybe making it clear in the docs that you *will* break compatibility in point releases and using `imdl` as a library is not something you're going to support might be enough to minimise the extra support burden?