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 1925 page.
foo.is_number == not foo.free_symbols; update docstrings

To know whether something can be evaluated to a real number with
precision, use is_comparable; to know if it can be a complex number
with precision use something like

all(i.is_comparable for i in foo.as_real_imag())

is_comparable should make the call to is_number.
Merge pull request #1990 from smichr/isnumber

is_number docstrings updated; equals and is_constant modified