Parser has no string allocation

[axunonb]

• Reducing GC pressure by avoiding temporary string assignments. Depending on the input string, GC is reduced by 50-80%.
• ParserSettings: All internal character lists are returned as List<char>.
• Internal character lists are cached in the parser for better performance
• Connected modifications
  • New performance tests for Parser
  • Placeholder property Placeholder?.Parent is renamed to Placeholder?ParentPlaceholder to avoid confusion with Format property Format?.Parent.
  • Placeholder has additional internal properties FormatterNameStartindex, FormatterNameLength, FormatterOptionsStartindex and FormatterOptionsLength

[codecov]

Codecov Report

Merging #187 (b0e40a4) into version/v3.0 (dea77a7) will decrease coverage by 0%.
The diff coverage is 99%.

@@             Coverage Diff             @@
##           version/v3.0   #187   +/-   ##
===========================================
- Coverage            95%    95%   -0%     
===========================================
  Files                49     49           
  Lines              2010   2019    +9     
===========================================
+ Hits               1909   1910    +1     
- Misses              101    109    +8     

Impacted Files	Coverage Δ
src/SmartFormat/Core/Parsing/Format.cs	83% <80%> (-<1%)	⬇️
src/SmartFormat/Core/Parsing/Parser.cs	93% <99%> (+<1%)	⬆️
src/SmartFormat/Core/Parsing/FormatItem.cs	61% <100%> (ø)
src/SmartFormat/Core/Parsing/LiteralText.cs	100% <100%> (ø)
src/SmartFormat/Core/Parsing/Placeholder.cs	73% <100%> (-24%)	⬇️
src/SmartFormat/Core/Parsing/Selector.cs	100% <100%> (ø)
src/SmartFormat/Core/Settings/ParserSettings.cs	100% <100%> (ø)
src/SmartFormat/SmartFormatter.cs	97% <100%> (ø)

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