add sinc differentiation rule

[stevengj]

for JuliaDiff/ForwardDiff.jl#466

[codecov-commenter]

Codecov Report

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@@            Coverage Diff            @@
##             master      #48   +/-   ##
=========================================
  Coverage          ?   93.75%           
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  Files             ?        2           
  Lines             ?      128           
  Branches          ?        0           
=========================================
  Hits              ?      120           
  Misses            ?        8           
  Partials          ?        0           

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