overhaul lexer/parser according to the ISO standard.

[ichiban]

No description provided.

[codecov]

Codecov Report

Merging #227 (52e080d) into main (72c2134) will increase coverage by 0.19%.
The diff coverage is 98.29%.

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
+ Coverage   96.92%   97.12%   +0.19%     
==========================================
  Files          21       21              
  Lines        6319     6375      +56     
==========================================
+ Hits         6125     6192      +67     
+ Misses        160      151       -9     
+ Partials       34       32       -2     

Impacted Files	Coverage Δ
engine/atom.go	97.50% <83.33%> (-2.50%)	⬇️
engine/parser.go	97.69% <96.97%> (+2.60%)	⬆️
engine/builtin.go	96.41% <100.00%> (+0.15%)	⬆️
engine/compound.go	100.00% <100.00%> (ø)
engine/float.go	90.90% <100.00%> (ø)
engine/lexer.go	100.00% <100.00%> (ø)
engine/stream.go	100.00% <100.00%> (ø)
interpreter.go	94.88% <100.00%> (ø)
... and 2 more

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