Get rid of fake TypeInfo's for TypedDicts

We currently create fake TypeInfos to store TypedDicts. This used to cause various issues, in particular, currently the type of a TypedDict "class object" in runtime context is () -> SomeTypedDict, where the return type is an instance of the fake TypeInfo.

We can probably fix latter by just constructing a reasonable type, but a more global solution would be to get rid of the fake TypeInfo entirely. It is not needed for anything and just complicates things. We will need to put some symbol node in the symbol table, so we would need a new SymbolNode subclass (or reuse TypeAlias, so a TypedDict definition A = TypedDict('A' {'x': int, 'y': int}) will be very similar to A = Dict[str, int]).

Inizia tracciando dove le definizioni di TypedDict creano TypeInfos fittizi e come i relativi simboli vengono memorizzati nella tabella dei simboli. Valuta una nuova sottoclasse di SymbolNode o una rappresentazione di TypeAlias, quindi verifica che i TypedDicts non richiedano più TypeInfos fittizi e che il loro tipo di oggetto-classe nel contesto di runtime sia ragionevole.