Rename MaterialSpring to Spring

[jverkoey]

[pingpongboss]

The reason this was named MaterialSpring is because Rebound's library already defines a Spring class. If I rename this to Spring, devs' IDEs will ask them to disambiguate. This is not a big deal, except if devs use both material-motion's Spring and rebound's Spring. In that case, they will be forced to use the full package prefix when referring to one of them (dev's choice which one).

[jverkoey]

Discussed in sprint planning: it's not super likely that people will be using Rebound and Material Motion's spring in the same file, especially if we drop the Rebound dependency.