symbolic expression plots do not use the generic variable processing pipeline

[jasongrout]

Right now, plots for symbolic expressions do not go through the generic codepath, but through the expression.plot() function in symbolic/expression.pyx. This specialized codepath changes how the arguments are handled, etc. I think any functionality in the special codepath ought to go in the generic codepath, or we should reduce the generic codepath quite a bit and just pass things off to symbolic expressions. Too much code duplication here has led to lots of little inconsistencies.

As an example, this works plot(x^2,0,x,5), but this doesn't: parametric_plot((x,x^2), 0,x,5), since the first uses the expression.pyx codepath, but the parametric plot uses the generic codepath.

CC: @burcin

Component: graphics

Issue created by migration from https://trac.sagemath.org/ticket/7641