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.

Fix cypclass new not going into temp (and try to incref new statements before happily leaking memory)