RFC: add evalpoly function, mirroring @evalpoly macro

As mentioned in #7146, since we have an `@evalpoly(z, c...)` it makes sense to also have a `evalpoly(z, c)`, analogous to Matlab's [polyval](http://www.mathworks.com/help/matlab/ref/polyval.html) but with the arguments in reverse order and the coefficients in ascending order by power of `z`.

We could alternatively mirror Matlab and store the coefficients in descending order, in which case we might as well swap the arguments and call it `polyval`.  However, this issue seems to have been discussed in vtjnash/Polynomial.jl#5, and the consensus seems to have been to reject Matlab's ordering, resulting in @Keno's [Polynomials.jl](https://github.com/Keno/Polynomials.jl) fork.