Store keys and values seperately

[BeastyBlacksmith]

I haven't thought through every implication of this, but I think this makes implementation of dictionary behaviour easier.

I implemented get, + and - with this and you get getindex for free then, so fixes #12 .

[thofma]

Yes, I agree that this is cleaner. Thanks!

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 99.57% // Decreases project coverage by -0.42% ⚠️

Coverage data is based on head (16484de) compared to base (711f1d6).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##            master      #15      +/-   ##
===========================================
- Coverage   100.00%   99.57%   -0.43%     
===========================================
  Files            1        1              
  Lines          207      238      +31     
===========================================
+ Hits           207      237      +30     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
src/Tally.jl	99.57% <100.00%> (-0.43%)	⬇️

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[thofma]

I pushed some small tweaks.

[thofma]

Thanks!