Add support for mirroring

[isovector]

This PR adds a mirror method to Object which mirrors an object across a hyperplane defined by a given normal.

Example:

let cutcube =
      difference
        [ rect3R 0 (-3, 0, -3) (0, 3, 0)
        , sphere 3
        ]
 in union
      [ cutcube
      , mirror (1, 0, 1) cutcube
      ]

[julialongtin]

yeah, you're going to have to fix rotation, AND expose this via scad for me to accept it.

[isovector]

@julialongtin PTAL. The arbitrary reflection axis is now implemented, though I'm going to push back against the scad stuff. I don't use that interface, nor do I have any plan to. If someone else would like to expose this functionality, I'd be all for it, but this PR provides useful functionality as it is.

[julialongtin]

After discussion with the OpenSCAD folks, this is more powerful than their mirror() construct. I have two asks with this one: could you rename this version reflect(), and could you implement a mirror() that does not include the original object in the output? no and no is a fine answer, don't hurt me for asking. :)

[isovector]

This version doesn't include the original object either --- notice the union in the example! Am I missing something?