Add some missing primal_value methods

[mohamed82008]

This fixes #46 and defines missing primal_value methods for other backends too.

[sethaxen]

Thanks! Can you add tests for these?

[codecov-commenter]

Codecov Report

Merging #47 (28b9bb5) into master (be3de03) will increase coverage by 0.00%.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master      #47   +/-   ##
=======================================
  Coverage   83.00%   83.01%           
=======================================
  Files           5        5           
  Lines         459      465    +6     
=======================================
+ Hits          381      386    +5     
- Misses         78       79    +1     

Impacted Files	Coverage Δ
src/tracker.jl	94.44% <66.66%> (-5.56%)	⬇️
src/forwarddiff.jl	100.00% <100.00%> (ø)
src/reversediff.jl	100.00% <100.00%> (ø)

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[sethaxen]

One last suggestion. Afterward, if all tests pass, I think it's ready to merge.