Requirements: whitelist or blacklist standard library symbols

There should be a requirement rule that allows a teacher to define a whitelist or blacklist of allowed standard library symbols that can be used in a project. Examples (from @alphalambda):

* Build a rectangle using "polygon" instead of "rectangle" (using "rectangle" not allowed)
* Use the included definition of "ellipse" (which uses "circle") but do not use "circle" anywhere else in your code, and then draw a bunch of ellipses
* Build a tangram using just the provided polygonal pieces (polygon and rectangle are not allowed except in the definition of the provided tangram pieces)

Another one I've used:

* Draw a complete picture using nothing but `arc`s and transformations.