Implement comparison between Nemo and Singular rationals

[fingolfin]

Consider this:

julia> Singular.QQ(5) == 5
true

julia> Singular.QQ(5) == Nemo.ZZ(5)
true

julia> Singular.QQ(5) == Nemo.QQ(5)
false

Ooops!

The latter is of course quite bad and misleading. The cause is a problematic fallback in AbstractAlgbebra:

function ==(x::NCRingElem, y::NCRingElem)
   fl, u, v = try_promote(x, y)
   if fl
     return u == v
   else
     return false
   end
 end

We should address that there (and indeed, we tried that last year and it fizzled out, but we probably should get back to that. See Nemocas/AbstractAlgebra.jl#1853, Nemocas/AbstractAlgebra.jl#1854, Nemocas/AbstractAlgebra.jl#1873)

But of course in the meantime, we also should add suitable comparison methods in Singular.jl

CC @fieker @hannes14