Simplify constants in powers and sums

[saschatimme]

This adds logic to simplify constant in order to get rid of silly things like

julia> Differential(x)(Differential(x)((x-2)^2)) |> expand_derivatives |> simplified_expr
(x-2) ^ ((2-1)-1) * 2

Now this just returns

julia> Differential(x)(Differential(x)((x-2)^2)) |> expand_derivatives |> simplified_expr
2

Also the third derivative would have errored, but now correctly returns 0.

I know that this will be replaced by smarter rewrite rules at some point but for now I thinks it's not bad to have this.

[codecov]

Codecov Report

Merging #159 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #159      +/-   ##
==========================================
+ Coverage   92.69%   92.84%   +0.14%     
==========================================
  Files          12       12              
  Lines         452      461       +9     
==========================================
+ Hits          419      428       +9     
  Misses         33       33

Impacted Files	Coverage Δ
src/simplify.jl	93.87% <100%> (+1.37%)	⬆️

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