Implement parallel e-graph search

[mwillsey]

This PR implements parallel e-graph search based on an egg API I just added. The new API doesn't do parallelism, just enables clients to do it themselves.

I have not measured the effects of the parallelism just yet.

[codecov]

Codecov Report

Attention: Patch coverage is 94.73684% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.54%. Comparing base (1c63de0) to head (d587167).

Files with missing lines	Patch %	Lines
...st/cubesql/cubesql/src/compile/rewrite/rewriter.rs	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8661      +/-   ##
==========================================
- Coverage   82.55%   82.54%   -0.02%     
==========================================
  Files         209      209              
  Lines       77054    77051       -3     
==========================================
- Hits        63612    63599      -13     
- Misses      13442    13452      +10     

Flag	Coverage Δ
cubesql	82.54% <94.73%> (-0.02%)	⬇️

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