Improve performance of derivative with FiniteDifferences and ForwardDiff

[devmotion]

Fixes #87. With this PR, the benchmark in #87 yields on my computer

julia> @benchmark with_AD(1.)
BenchmarkTools.Trial: 10000 samples with 960 evaluations.
 Range (min … max):  87.060 ns …  1.505 μs  ┊ GC (min … max): 0.00% … 88.90%
 Time  (median):     89.291 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   92.532 ns ± 30.948 ns  ┊ GC (mean ± σ):  0.75% ±  2.17%

  █▆▆▆▃▂ ▄▅▄▂▁▁▁ ▄▂▁▁                                       ▁ ▂
  ███████████████████▇▆▇▇█▇▇▇▇▆▅▆▅▅▅▅▄▄▃▃▄▄▁▆▃▁▃▄▃▃▅▃▃▁▁▄█▆▄█ █
  87.1 ns      Histogram: log(frequency) by time       138 ns <

 Memory estimate: 32 bytes, allocs estimate: 2.

julia> @benchmark without_AD(1.)
BenchmarkTools.Trial: 10000 samples with 961 evaluations.
 Range (min … max):  86.429 ns …  1.699 μs  ┊ GC (min … max): 0.00% … 91.77%
 Time  (median):     89.110 ns              ┊ GC (median):    0.00%
 Time  (mean ± σ):   91.506 ns ± 35.367 ns  ┊ GC (mean ± σ):  0.90% ±  2.23%

  ▃█▇▃▃██▄▄▂▁   ▂▃▄▄▄▂▁▁       ▃▂                             ▂
  ████████████▇██████████▇██▇▇████▇███▆▆▅▂▅▄▅▇█████▅▅▆▆▅▆▅▂▃▆ █
  86.4 ns      Histogram: log(frequency) by time       113 ns <

 Memory estimate: 32 bytes, allocs estimate: 2.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.13% 🎉

Comparison is base (041d760) 84.25% compared to head (8ba18a0) 84.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
+ Coverage   84.25%   84.38%   +0.13%     
==========================================
  Files           8        8              
  Lines         470      474       +4     
==========================================
+ Hits          396      400       +4     
  Misses         74       74              

Files Changed	Coverage Δ
ext/AbstractDifferentiationFiniteDifferencesExt.jl	100.00% <100.00%> (ø)
ext/AbstractDifferentiationForwardDiffExt.jl	100.00% <100.00%> (ø)

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