symbolic_solve introduces implicit constraints

[AayushSabharwal]

symbolic_solve sometimes chooses solutions which implicitly constrain the solution space. Consider the following two polynomials:

julia> polys
2-element Vector{Num}:
 -1 + x^2 + y^2
  y - z - (x^2)

symbolic_solve will give solutions which have sqrt(z^2 - 1) terms in them, implicitly constraining z >= 1 when [x => sqrt(3)/2, y => 1/2, z => -1/4] is a perfectly valid solution.