Make Attribute.get_value rw. Fixes #2521

[FCO]

No description provided.

[lizmat]

Currently, you cannot change the value of the attribute with get_value, only with set_value. This change would allow a container to leak out, even if the attribute was not marked is rw. And with this change that would only work for non-native (Opaque) attributes.

It is true that set_value doesn't respect the is rw setting on the attribute either.

In any case, it feels to me that either it shouldn't matter whether the attribute is a native attribute or not. Or that this PR should be rejected because if you want to be that low-level, you should use set_value if you want to set the value of an Attribute in an object that way.

[lizmat]

After some more thought today, I think we can consider Attribute.get_value and Attribute.set_value sufficiently low-level to take all of the safeties off. Working in that now.

[lizmat]

Thanks for the pointers. Fixed it more extensively with a5411e4