Use macro to generate most of the UntypedVal API

[Robbepop]

This will simplify design and implementation of #1389.

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 70.48%. Comparing base (bec7883) to head (b34fccd).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/core/src/untyped.rs	91.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1391      +/-   ##
==========================================
- Coverage   70.96%   70.48%   -0.49%     
==========================================
  Files         155      155              
  Lines       14683    14445     -238     
==========================================
- Hits        10420    10181     -239     
- Misses       4263     4264       +1     

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