Upgrade/peg

This is based on top of !4 (closed) to make use of current rust stable.

Peg 0.5 would create source code that triggers warnings with current rust stable.

Upgrade to peg 0.6.

This is a first take. There's a few open questions from my point of view:

• The return types in src/grammer/parser are getting long Would it help to define a type?

• The grammar used to be in a separate .rustpeg file The grammar syntax does not work well with autoindent. Peg itself still does so and has a bootstrap script. Is the cleaner separation worth the added build complexity?