Array wrapping

[Rich-Harris]

This syntax is minging:

state.set( 'data[3]', { foo: 'bar' });

// or even worse
state.set( 'data[' + state.get( 'data.length' ) + ']', { foo: 'bar' });

It would be much nicer to be able to use standard array mutator methods. We should wrap those methods so that a call to push is equivalent to the above.