Properly differentiate between *mut and *const

[konstin]

This is the definition of ToPyPointer:

pub trait ToPyPointer {
    fn as_ptr(&self) -> *mut PyObject;
}

With this approach, we implicitly cast from immutable to mutable, which should never be done. Instead ToPyPointer should have a as_mut_ptr and a as_ptr method.

[konstin]

Closing in favour of #342