Improve performance of recent function-index

[Tokazama]

Previous versions were trying to use firstindex and lastindex to reference an axis but first(axis) and last(axis) is what we actually want and is faster.

[codecov]

Codecov Report

Merging #319 (c26988e) into master (5228630) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #319      +/-   ##
==========================================
- Coverage   91.86%   91.79%   -0.07%     
==========================================
  Files           9        9              
  Lines        1439     1439              
==========================================
- Hits         1322     1321       -1     
- Misses        117      118       +1     

Impacted Files	Coverage Δ
src/indexing.jl	89.34% <100.00%> (-0.41%)	⬇️

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[Tokazama]

@ChrisRackauckas, I'm assuming those SciMLBase failures are unrelated?

[ChrisRackauckas]

Passed in the second run. Thanks for letting me know, that was a goof on my end.