* Make Variable::from_symbol public.
* Implement basic parsing of queries.
* Use pinned dependencies the hard way to fix Travis.
* Bump ordered-float dependency to 0.4.0.
* Error coercions to use ?, and finishing the find interface.
The parser mostly works and has a decent test suite. It parses all the
queries issued by the Tofino UAS, with some caveats. Known flaws:
* No support for tagged elements, comments, discarded elements or "'".
* Incomplete support for escaped characters in strings and the range of
characters that are allowed in keywords and symbols.
* Possible whitespace handling problems.
