Prohibit usage of void in local variable declarations and field declarations
Prohibit usage of void in parameter declarations
Add test for typechecking local variable declarations
Replace TypeBasic(...) with type constants
Allow assigning null to references only
Make Typer accept only a Program, not a SyntaxTree
Check return type of methods
Check for a return statement on every code path
Get rid of an Iterator (thanks @Kha)
Fix awfully stupid wrong typing of return statements
Convert more TypeBasic(...) to builtins, fix one match case
Merge pull request #100 from jspam/038-moar-typer

Extend the typer to typecheck many more things
Add tests for namer
Implement SyntaxTreeVisitor
Implement Namer