Fix State instance loosing it's context

[farskid]

The problem

Currently, since the matches and toStrings methods are added to the prototype of State, the context of this will be lost in case of reassignment or object destructing.

const { matches } = machineState;

console.log(matches('some_state'));

[davidkpiano]

Thanks @farskid!! Can you write a quick test?

[farskid]

@davidkpiano Tests added to the State test suite.