Fix issue #3738
Add test cases for issue #3738
Add other two test cases
simplify -1 and I handling in Mul.flatten

The best of 3 runs of the core test suite gave, in seconds,

master: 43, 48, 57
now: 36, 36, 45

So this appears to be a little faster and leads to a more compact result.
watch for unevaluated I**e
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 #1991 from mrocklin/remove-print

remove errant print statement
Merge pull request #1989 from lidavidm/issue_3738

Fix issue 3738
Merge pull request #1987 from smichr/n1

-1 and I flatten better in Mul.flatten
Merge pull request #1990 from smichr/isnumber

is_number docstrings updated; equals and is_constant modified
reify
__repr__ for unify.core classes
rewrites first commit
add test for strategy
tree traversal functions for unify/core
add logpy test
computations first commit
add edges
add CompositeComputation and unique function