make ∇ and Δ const bindings

[KristofferC]

Otherwise, using them will be slower than necessary.

[chakravala]

Thanks, by the way this is only a preliminary release, which will be integrated with other packages.

The full intended usage of this package will be explained once more testing and design is complete.

[KristofferC]

I undeestand, was just browsing around and saw a low hanging fruit :)

[chakravala]

Leibniz.jl will be designed to universally handle various interoperability tasks surrounding Leibniz differentials, gradients, and non-linear operators.

For example, the ∇ and Δ are Leibniz.Derivation similar to LinearAlgebra.UniformScaling, so that the intention of the symbols can be carried around into various contexts where information is needed.

The implementation will be separated from LinearAlgebra.UniformScaling soon, but it still uses it in the underlying type for quick initial implementation purposes.