Remove op clone

[jacwright]

I'm wondering why https://github.com/quilljs/delta/blob/master/lib/delta.js#L49 line (and the deep clone dependency) is needed and if it ought to be removed for better performance. I've not seen performance issues but saw this and thought it was unnecessary. Thoughts?

[jhchen]

It possibly modifies two lines after the pasted line so would not be a benign change to not clone it first. It is also used in op.js for similar reasons.

[jacwright]

Was just thinking that all the methods that use push are creating new objects, so it is unnecessary there. But if push is being used outside of the class (it is publicly available after all) then probably best not to treat it as immutable.