Switch to join-based PRISM category lookup

[dylanhmorris]

Closes #264

Also makes as_of potentially vectorized in cutpoint lookup, which is relevant for (e.g.) defining true categories for a timeseries of dates that spans cutpoint revisions.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.64%. Comparing base (56ac56c) to head (f2e33fe).

Files with missing lines	Patch %	Lines
R/categorize_prism.R	87.93%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #265      +/-   ##
==========================================
+ Coverage   75.51%   75.64%   +0.13%     
==========================================
  Files          37       37              
  Lines        2091     2131      +40     
==========================================
+ Hits         1579     1612      +33     
- Misses        512      519       +7     

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