Implement amendments and small fixes

[dgg]

• deprecate BGN for EUR
• add ReadonlySpan<char> overloads to Money.Parse() and Money.TryParse() (closes refactor Money.Parse and Money.TryParse #68)
• add analysis attributes to .Try*(..., out ...) methods to improve dev-ex (closes Add NotNullWhen to TryGet and Related Methods #79)

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.91%. Comparing base (845bc10) to head (8b3f856).

Files with missing lines	Patch %	Lines
src/NMoneys/Money.Parsing.cs	86.66%	2 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   74.10%   73.91%   -0.20%     
==========================================
  Files          62       62              
  Lines        1178     1177       -1     
  Branches      127      128       +1     
==========================================
- Hits          873      870       -3     
- Misses        285      287       +2     
  Partials       20       20              

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[coveralls]

Coverage is 74.681% — feat/amendments-improvements into master. No base build found for master.