Add ability to use array suffixes in JSON reform files

[martinholmer]

This pull request, which follows up #1503, implements the idea suggested by @talumbau in TaxBrain issue 596. This Tax-Calculator enhancement should enable a major simplification in TaxBrain logic and the elimination of at least one TaxBrain bug (the one discussed in issue 596).

See the three new test_translate_json_reform_suffixes_* test in the test_calculate.py file for details on how this enhancement can be used. The new capability is optional in the sense that all old JSON reform files continue to work as they did before. The new optional capability is meant primarily for TaxBrain, but could be used in hand-written JSON reform files.

@MattHJensen @feenberg @Amy-Xu @andersonfrailey @hdoupe @GoFroggyRun @codykallen
@talumbau @PeterDSteinberg @brittainhard

[codecov-io]

Codecov Report

Merging #1505 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1505      +/-   ##
==========================================
+ Coverage   99.92%   99.96%   +0.04%     
==========================================
  Files          37       37              
  Lines        2507     2553      +46     
==========================================
+ Hits         2505     2552      +47     
+ Misses          2        1       -1

Impacted Files	Coverage Δ
taxcalc/calculate.py	100% <100%> (ø)	⬆️
taxcalc/policy.py	100% <100%> (+1.14%)	⬆️

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