There's an unfortunate conflation here between implementing the query
parser in `rust-peg` and moving some validation that now happens at
parse time to happen later. The result is that we introduce
`ParsedFindQuery` as a less-processed `FindQuery`, and that we only
use string errors (which is all `rust-peg` supports) instead of the
structured errors in query-parser's errors module. The next commit
will address this, on the road to removing the `query-parser` module
entirely.
