This effectively allows stubs to be placed on the `new` method for types.
A strange issue arose when the keyword was allowed.
The compiler failed to resolve the Stub type from the Double initializer.
The error trace goes through null_double_spec.cr.
Running just that spec file confirms the issue is there, but running other individual files doesn't produce the error.
As a workaround, I've put the full path of Stub in the initializer.