Make Level.elm dumb. This is a less functional style, in a way, but it is crazy how much shorter it is, and no harder to read.
Update readme to the latest elm name for that thing
Give names to the magic numbers in enemyVelocity; instead of hardcoding 34, use Level.fleetSize.
Shorter definition of cubicEasing.
Make Utils.cubicEasing a prettier API, and make the two call sites use 1 as the final number of enemies rather than 0.