ENH: Parallel specification of next API

[effigies]

Trying out some Protocols to start on #989 and get some feedback.

The goal is to shift this into bids, but for right now it's easier to use a separate package.

Some initial notes:

1. Trying out putting get() back, but ensuring it only ever returns files.
2. I'm using Dataset instead of Layout. I think that will help make a cleaner mental break, and could allow us to have both implementations in parallel for a while.
3. We probably want to come up with different names for the protocols than the concrete classes. Will need to think about this. Open to suggestions.

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (22d2eb5) 83.42% compared to head (bcddeb1) 83.42%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #999   +/-   ##
=======================================
  Coverage   83.42%   83.42%           
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  Files          38       38           
  Lines        4290     4290           
  Branches     1100     1100           
=======================================
  Hits         3579     3579           
  Misses        515      515           
  Partials      196      196           

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