Moved the expressions to a subpackage.
Renamed 'Visitor' to 'ASTNodeVisitor' and combined 'Visitee' and 'ASTNode'.
Added the Declaration AST Node.
Moved the value types to a subpackage, and let them implement the 'Value' class.
Replaced the type as a string with object instances.
Made the parser in the parser tests final.
Added type checker shell.