Rewrite typechecker

[AZHenley]

The typechecker has become difficult to debug and extend.

Proposed rewrite:

• All statements evaluate to void.
• Don't differentiate between statements and expressions.
• One switch statement that handles all nodes.
• Simply perform a postorder traversal of the entire AST.

[AZHenley]

More:

• Helper functions to create the type objects
• Maintain a structure of primitive types
• Remove name checks (e.g.,is this equal to "int")
• Break code up into multiple files