Make `get_next()` accept a `processID` parameter

Make `get_next()` accept a `processID` as a parameter. `get_next()` now searches
for the next process relative to the given processID instead of the current
ringuffer index position.

Importantly, the only functionw which now affect the ringbuffer index are
`add_process()` and `set_current()`. The codebase now agrees with the
documentation laid out in xrndbuffer.h.
Add member `score` to struct `process`

Members "score" and "priority" can together be used to make modestly nuanced
decisions about scheduling. The end goal is this:

    1. Each time `Schedule()` is called, bump the score of each process. The
       more the score is bumped.
    2. Select the process with the highest score, reset the score to 0, and run
       it. If multiple processes have the same score, run the first process
       after the currently selected process.
Schedule processes using a scoring system

See previous commit message for concept
Clean up warnings

`run_idle` was defined but not referenced.

`candidate` and `current` have different types, so the statement `candidate =
`current` was invalid.
Add documentation

Write documentation for the roundbuffer and scheduler functions.