Add benchmark project and fix a few performance issues

[waf]

• Add benchmark project.
• Reduce memory usage for longer sessions (see comments near GetCompilationAsync)
• Re-order warmup to warm up per-keystroke operations first; evaluation warmup can run after that
• If code completion is not yet warmed up, don't open the completion menu -- that's responsible for about ~1s of lag when first typing in a newly started REPL.

[codecov]

Codecov Report

❌ Patch coverage is 85.10638% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.4%. Comparing base (4c2b550) to head (a8075d3).

Files with missing lines	Patch %	Lines
CSharpRepl.Services/Roslyn/RoslynServices.cs	84.7%	4 Missing and 3 partials ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #459   +/-   ##
=====================================
  Coverage   70.3%   70.4%           
=====================================
  Files         94      94           
  Lines       6805    6836   +31     
  Branches    1000    1005    +5     
=====================================
+ Hits        4789    4814   +25     
- Misses      1691    1694    +3     
- Partials     325     328    +3     

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