Move length function as a rewrite function

[andyfengHKU]

This PR moves another function using rewrite function framework. The ultimate goal is to remove if functionName == XXX check in the binder.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.70%. Comparing base (b491af2) to head (b609572).
Report is 43 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3442      +/-   ##
==========================================
+ Coverage   92.59%   92.70%   +0.10%     
==========================================
  Files        1205     1202       -3     
  Lines       44825    44867      +42     
==========================================
+ Hits        41504    41592      +88     
+ Misses       3321     3275      -46     

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