Codify the stages of compilation

[Mercerenies]

We currently have an informal compilation pipeline that looks something like

.lisp --> AST --> IR --> GDScript AST --> .gd

Document this process formally, so that as it inevitably gets more complex we can track it.

[Mercerenies]

There is now an additional stage that happens between IR and GDScript AST which checks for duplicate names (as specified in #41).

[Mercerenies]

Preliminary docs at https://github.com/Mercerenies/gdlisp/blob/master/doc/CompilationStages.md. Will be expanded as we move forward, but it's there, and that's the important thing.