Allow lowercase primitives and increase code coverage

[ryardley]

• Add lower case primitaves eg. string, number, etc.
• Add undocumented p. unsafe_tokens and p.unsafe_rpn for dedugging.
• Increase code coverage.
• Some tidy up

[codecov-io]

Codecov Report

Merging #44 into master will increase coverage by 9.82%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master    #44      +/-   ##
========================================
+ Coverage   90.17%   100%   +9.82%     
========================================
  Files           8      8              
  Lines         285    300      +15     
  Branches       40     40              
========================================
+ Hits          257    300      +43     
+ Misses         28      0      -28

Impacted Files	Coverage Δ
index.js	100% <100%> (ø)	⬆️
grammar.js	100% <100%> (+25.45%)	⬆️

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