fix crash with starred assignments where IndexNode generated for type inference ended up being visited

extend array assignment test to include unpacking and starred assignments (even though it's currently highly inefficient)

be a bit smarter about the item index during type inference of indexed slices to allow for better type inference of non-homogeneous containers

make item types of tuple/array unpacking inferable and institutionalise the IndexNode creation for the inference

(Seems we're missing tests...)

clean up recent incorrect change in Symtab code that validated the property generation for extension type attributes

--HG--
extra : amend_source : d2906987d10e7770ca09dde406219fb0d6a5d38c

allow signature return type annotations to provide a more specific return object type for def functions

--HG--
extra : amend_source : fe959a1a34b1e190fbe051c7058fcf51c50a2fa8