upgrade solving for abs, powers, and real/imag variables

Also:

    use powdenest before factoring in _tsolve

    fix detection of difference of functions
add bivariate solving to solvers
edits after coverage check
modify preprocessing for lambert
add recursive factor to namespace
remove use of match from _tsolve
add symmetric lambert detection
factor
fix tests and use rfactor

XFAIL a test that comes back with more than 1 constant; I don't
know why that one is failing.