Implement simple symbol table
Merge pull request #94 from Kha/simple-symbol

Implement simple symbol table
Modify syntax tree for naming and typing

 - Introduce new trait Typed for an AST element that has a type
 - Introduce new trait Ref for an AST element that references another
   AST element (more specifically, a Decl).
Make LocalVariableDeclStatement a TypedDecl
Merge pull request #97 from jspam/036-ast-changes-for-naming

Modify syntax tree for naming and typing
Implement the type(checke)r
Add test for correct return type