[CHORE] Return &str for expression name

[colin-ho]

We return a DaftResult<&str> for expression names, but currently there's no case that would raise an error. Having the return type as &str should be sufficient.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.59%. Comparing base (da991c5) to head (ac5ee6e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2224   +/-   ##
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  Coverage   85.59%   85.59%           
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  Files          71       71           
  Lines        7586     7586           
=======================================
  Hits         6493     6493           
  Misses       1093     1093           

[samster25]

LGTM