Update/remove unused code

[TimothyMakkison]

• Deleted the unused Poly1305Donna class.
• Deleted ByteIntegerConverterExtensions
• Removed unsed code from ByteIntegerConverter and replaced manual byte conversions with BinaryPrimitives methods.
• Converted Ed25519 methods to use Spans.

[codecov-commenter]

Codecov Report

Merging #93 (2a48ed0) into master (9637767) will increase coverage by 3.10%. The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   82.45%   85.56%   +3.10%     
==========================================
  Files         105      103       -2     
  Lines        5387     5141     -246     
  Branches      344      336       -8     
==========================================
- Hits         4442     4399      -43     
+ Misses        815      615     -200     
+ Partials      130      127       -3     

Impacted Files	Coverage Δ
src/Paseto/Cryptography/Ed25519.cs	49.09% <40.00%> (+4.82%)	:arrow_up:
...seto/Cryptography/Internal/ByteIntegerConverter.cs	100.00% <100.00%> (+75.69%)	:arrow_up:
src/Paseto/Cryptography/Sha512.cs	85.33% <100.00%> (ø)

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[daviddesmet]

Sorry for the late follow-up @TimothyMakkison I've just merged your changes, thanks a lot for all the contributions made to the project =)