This adds tests consisting of source code and a printout of its rewritten AST.

The test output becomes useless because we can't map the root node to anything at the moment

…n in field patterns

Two changes to parse_query_fields:

- Allow `field: (kind)* @cap` (repetition + optional capture) in field
  position, mirroring how it works for bare children.
- When the same field name is declared multiple times in a query (e.g.
  `condition: (foo) condition: (bar)`), merge them into a single
  ordered list of children rather than emitting duplicate field
  entries (which at runtime restart the iterator for the field and
  cause the second declaration to re-match from the first child).