Kill newcheck (request for comments)

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).