feat: data representations allow custom parsing and formatting of API fields.

See PR #2523. Most notable code changes:

- Load data representation casts into schema cache.
- Data representations for reads, filters, inserts, updates, views, over joins.
- `CoercibleField` represents name references in queries where coercion may be needed.
- `ResolverContext` help facilitate field resolution during planning.
- Planner 'resolves' names in the API query and pairs them with any implicit conversions to be used in the query builder stage.
- Tests for all of the above.
- More consistent naming (TypedX -> CoercibleX).

New: unit tests for more data representation use cases; helpful as examples as well.

New: update CHANGELOG with data representations feature description.

Fixed failing idempotence test.

New: replace date formatter test with one that does something.

Fixup: inadvertent CHANGELOG change after rebase.

Cleanup: `tfName` -> `cfName` and related.

Document what IRType means.

Formatting.

New: use a subquery to interpret `IN` literals requiring data rep transformation.

- With the previous method, very long queries such as `ANY (ARRAY[test.color('000100'), test.color('CAFE12'), test.color('01E240'), ...` could be generated. Consider the case where the parser function name is 45 characters and there's a hundred literals. That's 4.5kB of SQL just for the function name alone!
- New version uses `unnest`: `ANY (SELECT test.color(unnest('{000100,CAFE12,01E240,...}'::text[]))` to produce a much shorter query.
- This is likely to be more performant and either way much more readable and debuggable in the logs.

test/spec/fixtures/data.sql

@@ -839,3 +839,13 @@ INSERT INTO posters(id,name) VALUES (1,'Mark'), (2,'Elon'), (3,'Bill'), (4,'Jeff
 
 TRUNCATE TABLE subscriptions CASCADE;
 INSERT INTO subscriptions(subscriber,subscribed) VALUES (3,1), (4,1), (1,2);
+
+TRUNCATE TABLE datarep_todos CASCADE;
+INSERT INTO datarep_todos VALUES (1, 'Report', 0, '2018-01-02', '\x89504e470d0a1a0a0000000d4948445200000001000000010100000000376ef924000000001049444154789c62600100000000ffff03000000060005057bfabd400000000049454e44ae426082', '2017-12-14 01:02:30', 12.50); -- smallest possible PNG
+INSERT INTO datarep_todos VALUES (2, 'Essay', 256, '2018-01-03', NULL, '2017-12-14 01:02:30', 100000000000000.13); -- a number which can't be represented by a 64-bit float
+INSERT INTO datarep_todos VALUES (3, 'Algebra', 123456, '2018-01-01 14:12:34.123456');
+INSERT INTO datarep_todos VALUES (4, 'Opus Magnum', NULL, NULL);
+
+TRUNCATE TABLE datarep_next_two_todos CASCADE;
+INSERT INTO datarep_next_two_todos VALUES (1, 2, 3, 'school related');
+INSERT INTO datarep_next_two_todos VALUES (2, 1, 3, 'do these first');