* Test the mentat_query directory on Travis.
* Export common types from edn.
This allows you to write
use edn::{PlainSymbol,Keyword};
instead of
use edn:🔣:{PlainSymbol,Keyword};
* Add an edn::Value::is_keyword predicate.
* Clean up query, preparing for query-parser.
* Make EDN keywords and symbols take Into<String> arguments.
* Implement parsing of simple :find lists.
* Rustfmt query-parser. Split find and query.
* Review comment: values_to_variables now returns a NotAVariableError on failure.
* Review comment: rename gimme to to_parsed_value.
* Review comment: add comments.
Starting to work out the project layout for sub-crates. The crate inside query-parser/ is "datomish-query-parser" and the core code in src/ depends on it.
