Simplify the Haskell interface to rotateExtrude

[isovector]

The current public interface for rotateExtrude has a Maybe R parameter, representing whether or not we need to generate end caps for the shape, and if so, with what rounding. Unbeknownst to me, this parameter is automagically determined in the scad interface, meaning users of the Hs interface would need to decide for themselves whether to set the capping.

But this shouldn't be user-controllable at all, since getting it wrong completely breaks the mesh generation (see #307, #309, #310).

This PR moves the capping logic directly into getImplicit, where users can't screw it up. Furthermore, now that the rounding isn't given by a non-standard mechanism, we can instead use withRounding to set it.

While I was in the neighborhood, I added some optimizations to rotateExtrude so that we can skip it for the case of theta = 0 and when the object to extrude is emptySpace.

Fixes #307, fixes #308, fixes #309, fixes #310

[isovector]

After this lands, I'll make a golden test and PR for #288