Also misc cleanup:
- reorder members to be more consistent and fit the style guide
- remove use of _Pair
- made Variable.applyUpdate and Variable.owner public
- added docs to Priority, tweaked the code a bit
- added some docs to Result
- removed the internal-error Result (replaced with asserts)
- removed unused Results
- made Result const
- merged some files together since they had used privates a lot
I'm sorry this is completely unreviewable. I did the move from `lib/*`
to `lib/src/*` first, then did the `part`-to-`import` change, and then
found out how many of the files involved privates, which I wasn't
expecting. I can redo this as multiple commits if that would make it
easier to review.
552896afb7