Merge branch 'kill_newcheck'

Remove runtime checks on uninitialized attributes at the end of object
constructions.
Thus, uninitialized attributes will now fail (abort) on the first read
access.

Some people [citation needed] find that the current specification force
the usage of `nullable` on attributes that should not be null in
practice but that cannot be not nullable because the initialization is
not done in the constructor.

Some other people [bis] also state that the current check is problematic
with potential exception/longjump since uninitialized object could still
escape.

Therefore, because the next big devel planned is the constructors, I
prefer to solve this specification point before. This pull request is a
RFC on the new spec (and/or implementation).