Performance optimizations

[waf]

First commit: Better BenchmarkDotNet suite for evaluating changes.
Each subsequent commit shows performance improvements in the benchmark, and individually passes all tests.

abc4b93 is more aspirational then anything. It clearly is a better change but is more just a minor code cleanup than any sort of ground-breaking perf change.

[codecov]

Codecov Report

❌ Patch coverage is 84.84848% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.7%. Comparing base (e71a149) to head (2bb4a1a).

Files with missing lines	Patch %	Lines
src/PrettyPrompt/Highlighting/CellRenderer.cs	57.5%	11 Missing and 3 partials ⚠️
src/PrettyPrompt/Panes/CodePane.cs	68.7%	3 Missing and 2 partials ⚠️
...c/PrettyPrompt/Documents/StringBuilderWithCaret.cs	55.5%	0 Missing and 4 partials ⚠️
src/PrettyPrompt/Rendering/Screen.cs	50.0%	2 Missing and 1 partial ⚠️
src/PrettyPrompt/Pools.cs	96.8%	1 Missing and 1 partial ⚠️
src/PrettyPrompt/Documents/Document.cs	50.0%	1 Missing ⚠️
src/PrettyPrompt/Documents/WordWrapping.cs	96.4%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #284   +/-   ##
=====================================
  Coverage   84.6%   84.7%           
=====================================
  Files         47      47           
  Lines       3908    3992   +84     
  Branches     612     639   +27     
=====================================
+ Hits        3310    3383   +73     
- Misses       452     456    +4     
- Partials     146     153    +7     

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