Make declaration of cypclass methods work when declaring without defining a pointer type as return type

Hacky way to make const cypclass behave as const in Cython but not in C++ (useful for refcount bookkeeping for example)

Fix C++ garbage when trying to access __constructor__: generate each constructor wrapper as a static method inside the class

Remove dead code (checking for wrapper entry presence in cypclass, which is ensured by a default declaration during class declaration in symbol table)

(if the class redefines a constructor with a compatible signature,
it will get overridden, so it effectively prevents the default
constructor from spawning when there may be an ambiguity)

This reverts commit 5127550d.