Fixes issue8666 solvers
both lhs and rhs should be checked in solve
Merge pull request #8684 from aktech/issue8666

Solve returns correct solution for Equations where denominator becomes 0 (fixes #8666)
changes to solve and unrad
update solveset to use change variable of type matching solver
solveset: dispatch to radical solving; simplify inversion code
edit public docstring
8692: watch for TypeError during checking of candidates
delay collection of dens until needed during check
only solve up to quadratic in unrad
quick