Error-handling cleanup, etc.

[bdarcus]

Still not sure I want to merge all this, but am trying to better handle errors, and remove usage of unwrapand expect.

I'm particularly uncertain about the change from specific file parsing methods on the respective structs to a single one for all of them, with the new Parsable trait. I included this in the very large last commit. I may split off that change into a separate commit, so it's easy to revert later.

Also need to clean up/expand documentation.

[bdarcus]

Am just merging this, though it's not really finished.