[NEEDS TEST] Remove `Function` annotation to allow any callable type

[avik-pal]

[ChrisRackauckas]

add a test?

[codecov-commenter]

Codecov Report

Merging #981 (92479b5) into master (05d8646) will not change coverage. The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master     #981   +/-   ##
=======================================
  Coverage   77.32%   77.32%           
=======================================
  Files          26       26           
  Lines        3356     3356           
=======================================
  Hits         2595     2595           
  Misses        761      761           

Files	Coverage Δ
src/diff.jl	85.42% <100.00%> (ø)

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