Support update of multiple quantities in cart in a single mutation

[mlipscombe]

Is your feature request related to a problem? Please describe.
Currently, only a single CartItem can be updated at a time when calling updateItemQuantity. Given that it's a common operation to have a list of quantities and an "update cart" button on a shopping cart page, this is awkward.

Describe the solution you'd like
updateItemQuantity, or a new mutation like updateItemQuantities, should accept an array of CartItem keys and quantities, to allow for updating the cart quantities in a single mutation.

Describe alternatives you've considered
Multiple top level mutations, but this is really awkward in Apollo -- for example, using the <Mutation> component is a non-starter, and additional code is needed to generate multiple named mutations and then parse the results.

[kidunot89]

I have considered this, but haven't gone far beyond that. I do think is it's necessary however and will add an updateItemQuantities mutation in the next release.