WIP: improve performance

[mweststrate]

Should address concerns in #26 / #27.

Not entirely done, but results are very promising :)

    ✓ just mutate (3ms)
    ✓ deepclone, then mutate (417ms)
    ✓ handcrafted reducer (20ms)
    ✓ immutableJS (71ms)
    ✓ immer (proxy) - with autofreeze (118ms)
    ✓ immer (proxy) - without autofreeze (39ms)
    ✓ immer (es5) - with autofreeze (421ms)
    ✓ immer (es5) - without autofreeze (401ms)