Level transitions and scaling difficulty

This update is focused around adding difficulty scaling.

At this point, I've been playing the game quite a lot, to the point where I'm invincible.

This update made the game challenging again and added a milestone feature - level transitions:

Currently advancing through levels causes two notable things:

• Overall speed of the combo elements is scaled
• Some encounter become more difficult (e.g. more shurikens in each shuriken encounter)

These lay the groundwork for the next version's targets:

• End level rewards - you receive a reward (e.g. an extra heart or a slow motion trinket)
• End level passage picks - after finishing a level you get to pick the next level's reward, which will also effects its difficulty