Uncap minecart speed when going downhill

Not completely uncapped, probably diminishing returns when going up higher. This would allow the minecart to travel faster at the cost of having to go up first. Powered rails would still accelerate a minecart to the same speed as now, which would allow them to work as a ‘lift hill’ for the minecart. More speed for more setup time and recourses. This idea is heavily inspired by the way rollercoasters work.