Code cleanup

[TimothyMakkison]

• Converted remaining namespaces to file scoped namespaces
• Use compound assignment where available

[codecov-commenter]

Codecov Report

Merging #86 (1434ea3) into master (5ae908a) will increase coverage by 0.03%. The diff coverage is 80.87%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   82.59%   82.62%   +0.03%     
==========================================
  Files         104      104              
  Lines        5297     5295       -2     
  Branches      327      327              
==========================================
  Hits         4375     4375              
+ Misses        800      798       -2     
  Partials      122      122              

Impacted Files	Coverage Δ
src/Paseto/Cryptography/Blake2bMac.cs	67.74% <0.00%> (+2.11%)	:arrow_up:
...seto/Cryptography/Internal/Ed25519Ref10/keypair.cs	100.00% <ø> (ø)
src/Paseto/Cryptography/Internal/InternalAssert.cs	33.33% <0.00%> (ø)
src/Paseto/Cryptography/Internal/Poly1305Donna.cs	0.00% <0.00%> (ø)
src/Paseto/Validators/ExpirationTimeValidator.cs	41.17% <0.00%> (+2.28%)	:arrow_up:
src/Paseto/Serializers/JsonNetSerializer.cs	75.00% <75.00%> (ø)
src/Paseto/Cryptography/Internal/Sha512Internal.cs	100.00% <100.00%> (ø)

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