Cleanup codebase

[davidgoli]

• Exports datetime helper
• Replaces all usage of new Date(Date.UTC(..)) with datetime
• Converts dateutils from a namespace to a module

[codecov-commenter]

Codecov Report

Merging #520 (e89b5d2) into master (cb1dcfd) will increase coverage by 0.00%.
The diff coverage is 97.52%.

@@           Coverage Diff           @@
##           master     #520   +/-   ##
=======================================
  Coverage   88.69%   88.70%           
=======================================
  Files          29       29           
  Lines        2026     2027    +1     
  Branches      756      756           
=======================================
+ Hits         1797     1798    +1     
  Misses        229      229           

Impacted Files	Coverage Δ
src/cache.ts	92.72% <75.00%> (-0.26%)	⬇️
src/dateutil.ts	97.01% <96.96%> (+0.13%)	⬆️
src/datetime.ts	90.00% <100.00%> (ø)
src/datewithzone.ts	96.15% <100.00%> (ø)
src/index.ts	100.00% <100.00%> (ø)
src/iter/index.ts	97.22% <100.00%> (ø)
src/iter/poslist.ts	89.65% <100.00%> (ø)
src/iterinfo/index.ts	98.01% <100.00%> (ø)
src/iterinfo/yearinfo.ts	95.58% <100.00%> (ø)
src/iterset.ts	100.00% <100.00%> (ø)
... and 6 more

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