Extending the syntax of GRIN

[Anabra]

Ground work for the syntax extension

See #32

• [x] Added new AST definition
• [x] Added parser
• [x] Added pretty-printer
• [x] Added transformation from old to new
• [x] Added transformation from new to old
• [x] Added tests for old-to-new transformation
• [ ] Added tests for new-to-old transformation
• [x] Added roundtrip tests for old-new-old transformation

[andorp]

You could test the conversion with the existing grin files from the test directory.

[andorp]

After the review, we close this this PR, and open a new one from the 32-extended-syntax to a separate feature-branch.

[Anabra]

I will close this PR and open a new one to be merged into extended-syntax.