Capture slots

[nrxus]

Add an argument matcher that is able to capture an argument for later verification.

Potential pitfalls:

• It will probably require the argument to be cloneable + 'static. This is to help prevent the argument being dropped before the slot.
• It will probably require the slot to be surrounded by some sort of Rc<RefCell<T>>. This will help prevent any soundness issues with the slot being dropped before the last call to the mocked method.

cc: @muscovite