Refactor cast functions

[andyfengHKU]

Remove some if function name == CAST checking. The goal is to bind CAST as a regular scalar function. But some infrastructure is still missing.

[codecov]

Codecov Report

Attention: Patch coverage is 96.22642% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 93.26%. Comparing base (4692c54) to head (020c09b).

Files	Patch %	Lines
src/function/vector_cast_functions.cpp	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3016      +/-   ##
==========================================
- Coverage   93.26%   93.26%   -0.01%     
==========================================
  Files        1124     1125       +1     
  Lines       42953    42952       -1     
==========================================
- Hits        40061    40060       -1     
  Misses       2892     2892              

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