Semantic Passes

• BuildScopes? Add a scope to all ast-nodes. It also adds identifiers to it's own scope

• BuildSymbols? Assign a symbol to all decls.

• BuildTypes? Gives all exp's a type. In case of "errors", it will give a message and assign the DInt-type.

• ScopeCheck? Checks that the scope is intact: All identifiers that are used, hav been defined. All types used, have been defined.