* Pre: make FindQuery, FindSpec, and Element non-Clone.
* Pre: make query translator return a Result.
* Pre: make projection return a Result.
* Pre: refactor query parser in preparation for parsing aggregates.
* Pre: rename PredicateFn -> QueryFunction.
* Pre: expose more about bound variables from CC.
* Pre: move ValueTypeSet to core.
* Update some dependencies.
* Update rusqlite to 0.12.
* Update error-chain to a forked version that implements Sync.
* Fix some compiler warnings.
* Remove unused imports in tests.
* Parse errors no longer naturally print with the expected symbol.
This version removes nalexander's lovely matrix code. It turned out
that scalar and tuple bindings are sufficiently different from coll
and rel -- they can directly apply as values in the query -- that
there was no point in jumping through hoops to turn those single
values into a matrix.
Furthermore, I've standardized us on a Vec<TypedValue>
representation for rectangular matrices, which should be much
more efficient, but would have required rewriting that code.
Finally, coll and rel are sufficiently different from each other
-- coll doesn't require processing nested collections -- that
my attempts to share code between them fell somewhat flat. I had
lots of nice ideas about zipping together cycles and such, but
ultimately I ended up with relatively straightforward, if a bit
repetitive, code.
The next commit will demonstrate the value of this work -- tests
that exercised scalar and tuple grounding now collapse down to
the simplest possible SQL.
