lib: support iterator and asyncIterator

[JacksonTian]

No description provided.

[codecov-commenter]

Codecov Report

Merging #154 (d2038e2) into master (e35794d) will decrease coverage by 0.06%.
The diff coverage is 89.47%.

❗ Current head d2038e2 differs from pull request most recent head e84c755. Consider uploading reports for the commit e84c755 to get more accurate results

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
- Coverage   96.26%   96.21%   -0.06%     
==========================================
  Files          12       12              
  Lines        2516     2535      +19     
==========================================
+ Hits         2422     2439      +17     
- Misses         94       96       +2     

Files Changed	Coverage Δ
lib/util.js	100.00% <ø> (ø)
lib/semantic.js	93.76% <60.00%> (-0.13%)	⬇️
lib/lexer.js	100.00% <100.00%> (ø)
lib/parser.js	98.74% <100.00%> (+0.01%)	⬆️