Allow functions as input to fns generated by AD

[sritchie]

Fixes #189, thanks to @reedho for noticing this.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.38%. Comparing base (53fd990) to head (6d847cf).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/emmy/dual.cljc	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #190      +/-   ##
==========================================
- Coverage   87.44%   87.38%   -0.06%     
==========================================
  Files         100      100              
  Lines       15812    15812              
  Branches      849      853       +4     
==========================================
- Hits        13827    13818       -9     
- Misses       1136     1141       +5     
- Partials      849      853       +4     

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