PPB v0.7: Asset Rewrite

This updates for PPB v0.7 and Mutant Standard Emoji v0.4.1

PPB v0.7 dramatically altered how images are loaded via its new Asset system, and ppb-mutant was rewritten to be based on it.

• MutantSprite is now deprecated. Just use ppb.Sprite with image = Emoji(...)
• Added Emoji, which is an asset that takes the shortcode, morph, and tone.
• Added MorphToneGroup, which is a way to change the morph and tone of a whole group of Sprites at once.

All the internals were also updated for the VFS system.