Major update to allow ImplicitPlane objects to be created
automatically by the Parser when it sees equal signs. This makes it
possible to use ImplicitPlanes in Lists. (Before, when the List was
turned into a Formula returning a List, the equality became a plane
old equality rather than an ImplicitPlane object.)
This is a bit of a hack for now until I can make List and other Value
objects be allowed to have Formulas as their entries rather than turn
into Formulas returning Lists (or whatever type).