Support specializing on functions

[j-fu]

See Performance Tips .

In many places this was not implemented. This in particular would fix #516.

[codecov-commenter]

Codecov Report

Base: 87.29% // Head: 87.28% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (3dbf735) compared to base (76335e6).
Patch coverage: 96.55% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
- Coverage   87.29%   87.28%   -0.02%     
==========================================
  Files           9        9              
  Lines         913      912       -1     
==========================================
- Hits          797      796       -1     
  Misses        116      116              

Impacted Files	Coverage Δ
src/hessian.jl	97.56% <90.90%> (-0.06%)	⬇️
src/derivative.jl	94.87% <100.00%> (ø)
src/gradient.jl	98.92% <100.00%> (ø)
src/jacobian.jl	99.33% <100.00%> (ø)

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[KnutAM]

I'm not sure what the release plan for v0.11 is, but if not coming out soon, is backporting this fix to 0.10 desired?
(I can re-implement it on that branch if desired, I don't know if there is an easier way to do the backport)