Support complex pullbacks

[sethaxen]

Replaces _dot with _realdot, which allows for pulling back through functions with complex outputs. Resolves #22. This is currently untested.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.10%. Comparing base (b4343f5) to head (9bab121).
⚠️ Report is 66 commits behind head on master.

Files with missing lines	Patch %	Lines
src/AbstractDifferentiation.jl	16.66%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage   78.26%   77.10%   -1.16%     
==========================================
  Files           1        1              
  Lines         345      345              
==========================================
- Hits          270      266       -4     
- Misses         75       79       +4     

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[mohdibntarek]

Could you please add a test here for the intended use case?

[sethaxen]

Certainly, though the tests need a few refactors first to make testing these cases more straightforward. Some changes made in #27. More will be necessary.

[sethaxen]

Also, this PR should use RealDot.jl now that it exists.