Redesign bikes so they don't weigh three tons, and other sanity improvements

As mentioned in #5172

Currently, bikes are made from the same parts as cars, causing them to weigh a ton, and making the footpedal engine massively overpowered so it can move one.  Trying to fix that.

First go at changing bikes.  Values and items are a WIP, this is just to
give us something to work from.  Current status is:

New "lightframe" parts, made from a pipe, stats are 10% of normal
frames.

"box" part is also made with a pipe, cargo reduced 90%

bikes, carts, and swivel chair now use light parts

This is going to need a LOT of tweaking, please comment your opinion!