Support batch axes / broadcasting

[brentyi]

Makes it easier to:

• Use jaxlie with fewer vmaps.
• Make useful ports of jaxlie to numpy, torch, etc.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.79%. Comparing base (7374df2) to head (310320d).

❗ Current head 310320d differs from pull request most recent head f2eef4e. Consider uploading reports for the commit f2eef4e to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   99.81%   99.79%   -0.02%     
==========================================
  Files          13       13              
  Lines         546      496      -50     
==========================================
- Hits          545      495      -50     
  Misses          1        1              

Flag	Coverage Δ
unittests	99.79% <100.00%> (-0.02%)	⬇️

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