Avoid using Linq as it generates GC each time.

[karljj1]

I was doing some performance comparisons between our old version (2.x) and after upgrading to the latest version.
Previous formatting of a simple string such as "Hello {0} world {1}. This is a {0} test." would generate 0.6KB of garbage each run(after warmup).
When running with the latest version it was 2.6KB.
I found that most of this was coming from Source.cs HasNullableOperator. Its uses Linq which generates GC each time. So I have removed the usage of Linq in some of the critical parts. The GC is now down to around 0.5KB for us.

I also made the Source.cs method TryEvaluateSelector abstract as it seemed to make more sense.

[codecov]

Codecov Report

Merging #296 (4976725) into main (ed610a9) will decrease coverage by 0%.
The diff coverage is 100%.

❗ Current head 4976725 differs from pull request most recent head 90d6660. Consider uploading reports for the commit 90d6660 to get more accurate results

@@         Coverage Diff         @@
##           main   #296   +/-   ##
===================================
- Coverage    96%    96%   -0%     
===================================
  Files        91     91           
  Lines      3178   3184    +6     
===================================
+ Hits       3061   3066    +5     
- Misses      117    118    +1     

Impacted Files	Coverage Δ
src/SmartFormat/Core/Parsing/Format.cs	96% <100%> (+<1%)	⬆️
src/SmartFormat/Core/Sources/Source.cs	100% <100%> (+9%)	⬆️
src/SmartFormat/Extensions/ListFormatter.cs	98% <100%> (+<1%)	⬆️
...rc/SmartFormat/Extensions/LocalizationFormatter.cs	95% <0%> (-5%)	⬇️

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[axunonb]

Very good points - excellent PR 🎉
Yes, with the tiny input format string the effect for speed and GC is most significant, while for other input formats it's not that much (around 5-20%).