Make associative containers nothrow movable

[Thomas1664]

Although the standard doesn't require this, it would be nice if this implementation would make them nothrow movable anyways. This would allow vector<Associative Container>::push_back() to move on resize instead of copy.

[frederick-vs-ja]

I'm afraid that we can't do this at this moment due to ABI compatibility, see #1036.

(Perhaps the reason why the standard doesn't require this is keeping ABI compatibility.)