Small fixes for ExactReal

[Kolaru]

This contains two small fixes that are useful for the update of IntervalRootFinding.

1. Allow to use ExactReal{<:Integer} to index an array.
2. Fix derivative of ^ when mixing ExactReal and Interval in the ForwardDiff extension.

The first one in particular may be handy for everyone, when working with vectors of interval for example. With this PR, indexing work as intuitively expected with @exact

julia> @exact f(x) = x[1] + x[2] + 1.2
f (generic function with 1 method)

julia> X = [interval(-1, 1), interval(-0.1, 3)]
2-element Vector{Interval{Float64}}:
 [-1.0, 1.0]_com
 [-0.100001, 3.0]_com

julia> f(X)
[0.0999999, 5.20001]_com

[OlivierHnt]

Nice ! LGTM