Improve type stability and performance

[oschulz]

No description provided.

[codecov]

Codecov Report

Merging #9 (7b0930d) into main (c609e36) will increase coverage by 0.54%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   85.52%   86.07%   +0.54%     
==========================================
  Files           6        6              
  Lines         152      158       +6     
==========================================
+ Hits          130      136       +6     
  Misses         22       22              

Impacted Files	Coverage Δ
src/dual_numbers.jl	80.00% <100.00%> (ø)
src/flatten.jl	79.10% <100.00%> (ø)
src/fwd_back.jl	86.27% <100.00%> (+0.56%)	⬆️
src/rrules.jl	100.00% <100.00%> (ø)
src/with_forwarddiff.jl	100.00% <100.00%> (ø)

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