Add an AST visitor module and refactor the code generation to use it.

[jvprat]

This allows code reuse when navigating the AST, and it allows new modules that handle a few node types to be much smaller and simpler (I'm working on a new module).
In addition, the code generation and the checker are updated to use it, and some funcionts are split into smaller functions that handle concrete node types.

[mascarenhas]

It seems like the visitor module's default visitor would make it simpler to implement visitors that only need to act on specific AST nodes, correct? Could you do a new pull request with just the visitor module, without converting the existing code generator and type checker? Thanks!

[andremm]

@mascarenhas can we close this after merging PR #115 ?