.NET 7 Generic math support

[tompazourek]

• Add support for .NET 7 generic math #47

TODO

• Span-based TryFormat
• Span-based Parse and TryParse
• % operator
• TryConvertFrom* and TryConvertTo* methods
• Tests

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 54.63%. Comparing base (2aef85d) to head (3480d69).
Report is 116 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Rationals/Parsing.cs	0.00%	5 Missing ⚠️
src/Rationals/Operators.cs	0.00%	2 Missing ⚠️
src/Rationals/Formatting.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   55.17%   54.63%   -0.54%     
==========================================
  Files          17       17              
  Lines         812      820       +8     
  Branches      181      181              
==========================================
  Hits          448      448              
- Misses        330      338       +8     
  Partials       34       34              

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