Is_unit and is_nilpotent (with tests)

[JohnAAbbott]

Fixes #11

[thofma]

I don't think we can rely on any order when iterating over the coefficients. The only guarantee that we have is that coefficients and exponent[_vectors] have the same order. E.g., this is the implementation of constant_coefficient:

function constant_coefficient(p::MPolyRingElem{T}) where T <: RingElement
   if !iszero(p)
      for (c, v) in zip(coefficients(p), exponent_vectors(p))
         if iszero(v)
            return c
         end
      end
   end
   return zero(base_ring(p))
end

[JohnAAbbott]

I notice that there are tests of is_unit for Laurent series: for instance

./generic/LaurentSeries-test.jl:156:   @test is_unit(-1 + x + 2x^2)

Should I disable these tests? I have not yet checked which code would be called...
Perhaps it is simpler to leave things be?

[JohnAAbbott]

Right now I don't know how to write a test which will cover the lines which the codecov bot complained about. I don't think it is important to cover them.

[fingolfin]

@JohnAAbbott seems you and the code disagree as to what the correct output is?

[fingolfin]

Thepolynomial_ring should be over the ring created before, not over QQ. Fixed now