Move SpecialFunctions to an extension

[jishnub]

This should address the primary concern raised in #94

[codecov]

Codecov Report

Attention: Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 51.85%. Comparing base (85bc6b2) to head (6e67ff5).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/DualNumbersSpecialFunctionsExt.jl	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #100      +/-   ##
==========================================
+ Coverage   50.94%   51.85%   +0.90%     
==========================================
  Files           2        3       +1     
  Lines         212      216       +4     
==========================================
+ Hits          108      112       +4     
  Misses        104      104              

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