Carts: Merge boost_cart as "carts" mod

Carts: Merge boost_cart as "carts" mod

This is all the working code from SmallJoker's boost_cart, poored into
a more suitable form for minetest_game.

- Mesecons and moreores stuff was removed entirely.
- Textures were all renamed and moved out of default/
- Updated license, readme.txt, attribution
- Changed code license to MIT, left artwork at CC0
- removed default:rail and made aliases for it
- :carts:rail is now carts:rail.
- localized entity def
- removed copper rail entirely
- startstop rail was removed, as well as detector rail
- remodeled to b3d using stujones11 excellent blend model, but sizes
  of cart adjusted to make pixel sizes consistent (0.625) everywhere.
- slightly more complex texture map for the cart (front/side visibly
  different)
- rail parameters are passed as a separate def table, and stored in
  a private list. This avoids having to call `get_meta` on every
  node. In return, we need the node name, though.
- adds metal sounds (based on default metal sound function) and
  cart moving sound.
- reduced cart speeds to max 7, 5 by pushing.
- Added on_step() rail event handler, gets called when a cart is on
  a rail.
- Added various rebased updates from upstream (thanks Krock)
- Included a fix that removes the 'reverse jiggle' when stopping.
- Included reworked textures by sofar.

The mod namespace is still public, but I'm NOT declaring it an API. I'd
rather see it localized instead, for now. Any public interface in this
code is *experimental* at best, and should be considered non-stable and
unsupported for now.