Performance improvements

This release makes Shifty faster. It significantly reduces memory overhead by reusing objects and variables instead of re-declaring and initializing them.

The only functional change is trivial, but may potentially result in harmless warnings in some environments (specifically Node): e277dc7

If you see a warning in Node, simply add a .catch() to the Tween Promise or wrap it in a try/catch (which is a good practice for Promises generally).

Diff: v2.12.0...v2.13.0