Make all file writes generic

[Serial-ATA]

This adds two new traits, Truncate and Length, which are two operations we needed
that required the use of File.

With these two traits, combined with std::io::Read, std::io::Write and std::io::Seek, other types can now be treated as files.

It is now possible to write to a Cursor<Vec<u8>>.

TODO: changelog entry

[milesegan]

Are you interested in any help getting this one over the finish line? It would be great to be able to use lofty in wasm/web.

[Serial-ATA]

Are you interested in any help getting this one over the finish line?

Aside from me having to rebase this since its months old, it actually works just fine. I originally kept this as a draft due to the insane number of repetitive where clauses I had to add just to get it working.

I was hoping to find some kind of hack to clean this up, but in the end the only way is just waiting for Implied Bounds to land (so probably 6 years from now 🫤).

I haven't really looked at this in awhile, but since there's a handful of reactions on it I'll go ahead and merge it in as-is. I'll just need a day or two to bring the branch up to date.

[milesegan]

Thanks! I'm happy to help out with any grunt work if that's useful.

Thanks again for all your great work on lofty!

[Serial-ATA]

Dealing with all those merge conflicts was painful, I won't let PRs sit like this anymore 😄.

This'll be out in 0.19, still a few things I need to do before release.