First version of an AST

First, non-working version of an AST. This breaks the build!
Don't merge yet, I need to work on it more. Pushing this code
for initial feedback and review.

* Introduct an AST data structure
* Prototype modify parser to produce an AST for expressions
Fix the build
Merge branch 'ast' of https://github.com/dionyziz/llama
Hide implementation of scope stack.