Add compatibility with other mathematical software

Thanks for all the work in Reduce.jl, it is a great tool.
I need to complement it with Polymake.jl (to study some properties of the Newton polytope of a polynomial,
it is for [CRNT.jl](https://github.com/LauraBMo/CRNT.jl).
So, I need the exponents of a polynomial as Julia vectors of integers, and ideally another Julia vector, with the same order, containing the coefficients.
For this, I started with the exponent of a term
```
function extractexponentsofterm(T::RExpr)
return v->Algebra.deg(T,v)
end

julia> z=R"2*x*y^2+4*x^2-1"

2 2
4*x + 2*x*y - 1

julia> extractexponentsofterm(Algebra.lterm(z,:y)).([:x,:y,:z])
3-element Array{RExpr,1}:
1
2
0
```
But I am not sure how to iterate over all the term of a polynomial, and how to extract the coefficient neither.
One can iterate over `z:=z-lterm(z)` but requiring arithmetic operations seems not optimal.

Inspect Reduce.jl's Algebra.lterm and Algebra.deg entry points and the existing representation or traversal of polynomial terms; the issue does not name an implementation file. Done means exposing exponent vectors and same-order coefficient vectors for a polynomial without requiring repeated subtraction, including the shown z expression.

Written by the indexing model from the issue text.