Improve JPEG CQ

[iamcarbon]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

• Use span helpers (Fill & Copy) in PdfJsHuffmanTable
• General code cleanup.

[iamcarbon]

Ready for review.

[codecov]

Codecov Report

Merging #537 into master will decrease coverage by <.01%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master     #537      +/-   ##
==========================================
- Coverage   87.68%   87.67%   -0.01%     
==========================================
  Files         838      838              
  Lines       34698    34684      -14     
  Branches     2534     2533       -1     
==========================================
- Hits        30424    30410      -14     
  Misses       3524     3524              
  Partials      750      750

Impacted Files	Coverage Δ
...ts/ImageSharp.Tests/Formats/Jpg/JFifMarkerTests.cs	100% <ø> (ø)	⬆️
...s/Jpg/ReferenceImplementationsTests.AccurateDCT.cs	100% <ø> (ø)	⬆️
...p.Tests/Formats/Jpg/JpegImagePostProcessorTests.cs	67.92% <ø> (ø)	⬆️
...sts/Formats/Jpg/Block8x8FTests.CopyToBufferArea.cs	0% <ø> (ø)	⬆️
...s/ReferenceImplementations.GT_FloatingPoint_DCT.cs	100% <ø> (ø)	⬆️
...ils/ReferenceImplementations.StandardIntegerDCT.cs	93.68% <ø> (ø)	⬆️
...ests/Formats/Jpg/Utils/ReferenceImplementations.cs	65.3% <ø> (ø)	⬆️
...ImageSharp.Tests/Formats/Jpg/Utils/LibJpegTools.cs	86.79% <ø> (ø)	⬆️
...s/ImageSharp.Tests/Formats/Jpg/JpegEncoderTests.cs	100% <ø> (ø)	⬆️
.../ImageSharp.Tests/Formats/Jpg/LibJpegToolsTests.cs	82.35% <ø> (ø)	⬆️
... and 25 more

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[JimBobSquarePants]

This is a micro-optimization but remember span is slower than array on older platforms so we should be careful not to reduce throughput.