modify equals and is_constant

** Inline notes describe why the minimal_polynomial is not being used
at present to prove an nsimplify result. solve, however, may be able
to prove that an expression is zero by getting a self-consistent solution
when solving for a surd of the expression. An test was added for this.

** In addition, cancel is used on the expression to try facilitate
simplification to zero (and an expression that motivated the change
has been included).

** don't use equals in sign and abs because it can be very slow

Thanks to Mario Pernici for finding good examples of when the
method used in equals is going to take a long time to solve.
The discussion that I had with him is on the PR page.
define _addsort/_mulsort to identify its usage in codebase
3730: use as_content_primitive and cancel

* also change the general expand to only _mexpand when computing expr2
  in simplify
factor_terms can remove radical from +/- base
simplify handling of -1 and I in Mul.flatten
mod handles more simplification
geometry: reflect method; ellipse & RegularPolygon are oriented

** Now geometric objects can be reflected across a line.

This is not implemented for the Ellipse except in the case of horizontal
and vertical lines since arbitrary rotation of the ellipse is not supported.

** Now RegularPolygon and Ellipse track their orientation so they, like
Polygon, can yield a positive or negative area.

** use simplify in geometry

The only place where nsimplify is used is in the definition of Points.
That instance is now limited only to converting floats to Rationals.
Without the rational=True option it would have converted, with approximation,
expressions like follows:

>>> nsimplify((1/(exp(3*pi*I/5)+1)),rational=0)
1/2 - I*sqrt(sqrt(5)/10 + 1/4)

Although that is correct in this instance, it might not always be
correct.