make_function_plotter uses Numbas.jme.makeFast

The new implicitcurve object in JSXGraph 1.7.0 needs to evaluate its
function thousands of times for each update. It became clear that the
standard way of evaluating JME expressions is far too slow, so I wrote
`Numbas.jme.makeFast` to turn most expressions into almost-native JS
functions.

This commit changes the `make_function_plotter` code, which is used
whenever you use a subexpression as a parameter in a JSXGraph diagram,
to use `Numbas.jme.makeFast`.

It also makes `make_function_plotter` pick up the names of objects
already defined in the board, so you can use the values of things like
sliders in expressions.
If an object in the board has a `.Value` method, then you can use its
value by referring to it by its ID in the expression.

Example:

    jsxgraph(400,400, [-4,4,4,-4], [
        "a": ["slider", [[-3.5,2.5],[-1.5,2.5],[0,1,3]], ["id": "a"]],
        "f": ["implicitcurve", [expression("x^2-y^2-a")], ["id": "f"]]
    ])