default_init: error if collected attributes and inherited complex constructors

This unify the fate of collected attributes.
In the introduction of a class without explicit constructors, collectable
attributes are always considered in the providing of construction mechanisms.

This will also prevent programs that will be not exprimable with the first
version of new constructors.

Signed-off-by: Jean Privat <jean@pryen.org>
default_init: can compile combined init with collected attributes

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add base_init_combine.nit

Signed-off-by: Jean Privat <jean@pryen.org>
default_init: fix a NitUnit in template

Signed-off-by: Jean Privat <jean@pryen.org>