Merge branch 'expression' into develop

* expression:
  Return a ParsedResult class instead of raw AST node
Corrected example for negative indexing in README.

An earlier example indicates that `foo.bar[0]` returns "one".  Logic follows that `foo.bar[-1]` should also return a scalar value and not an array of values.
Merge branch 'trevorrowe-develop' into develop

* trevorrowe-develop:
  Corrected example for negative indexing in README.
Initial commit of jep-3
Allow projections on functions
Support type checking in argspec
Add support for variadic function

Used in not_null, which is variadic.
Final compliance tests
Fix bugs in sort_by