support Singleton type

[Roger-luo]

right now when we write

@basic(typeinfer=True)
def main(x):
    x = x + 1

the return type will be types.Literal(None) which is due to https://github.com/QuEraComputing/kirin/blob/main/src/kirin/dialects/func/typeinfer.py#L28 and the real reason we have this is because our type system does not understand singletons