Define AST structure

[Lúcás Meier]

Before we implement the parser, it would be a good idea to create a structure for our AST.

This is somewhat non-trivial, since we want to do the Roslyn approach of having a raw node type with the span information, and then overlay our syntax tree over that.

We can probably avoid doing anything fancy with arenas, but an optimization in the future might be to use arenas, since we have a uniform node type.