Tighten up parsing constraints

Change parser to disallow extra text at end of source.

Critical Bug fix

Fixes bug when backtracking to a deeply embedded varargs context.

need `dataclasses` to run on Python 3.6

No changes to Jertl code. Just the addition of a conditional package dependency on dataclasses which is required to run in 3.6

Initial release

Version 0.1.0 contains a basic but still practical package for working with structured data.
Operations are specified using a mini-language with a syntax which greatly resembles data's printed representation.