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This is a draft.  There's a LOT of hard design choices which are not
yet completely settled here.

Corresponding updates to the ipld/specs repo will be coming soon:
these implementations match a very recursive concept of selectors.
(https://github.com/ipld/specs/pull/116 is roughly this.)

There's so many things to go over, here.

- selectors at all
- selectFields is kind of a special case of unions
- selectPath as a concept is gone; is a degenerate case of selectFields
- we've added some new methods to help narrow down search spaces
- we have serial selector spec parsers...!  (very hand-rolled.)
- the idea of always reporting candidates vs definite match parents
  is dropped for now; it requires a different algo with different
  performance characteristics, so, we'll do it as such -- later.
- we ended up with unions being *in*!  They're going to be used
  internally by a bunch of other things -- see comment blocks in
  the code about that (tl;dr recursives imply a need of unions).

More docs will be coming in future commits; this is just a checkpoint
of the progress.
Signed-off-by: Eric Myhre <hash@exultant.us>