Formatting tool

[fjl]

Geas should have a formatting tool like gofmt. There are two parts to implementing this:

• Tracking comments in the AST: the parser has to be changed to attach comments to statement nodes. We need this to ensure the AST is a complete representation of the source code.
• AST writer: a function that writes out an AST using the canonical formatting

The formatting tool is also a precursor to stack analysis (#3).