Collapse AsSpan().Slice(..) calls into AsSpan(..)

[turbedi]

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 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) (N/A)

Description

This PR collapses all AsSpan().Slice(..) calls into AsSpan(..) (excluding test code), as it is less code and potentially a bit faster.

[CLAassistant]

All committers have signed the CLA.

[antonfirsov]

LGTM, thanks!

[codecov]

Codecov Report

Merging #1815 (10750fc) into master (6f0e825) will increase coverage by 0.00%.
The diff coverage is 80.00%.

@@           Coverage Diff           @@
##           master    #1815   +/-   ##
=======================================
  Coverage   87.09%   87.10%           
=======================================
  Files         936      936           
  Lines       48015    48015           
  Branches     6034     6034           
=======================================
+ Hits        41820    41824    +4     
+ Misses       5192     5189    -3     
+ Partials     1003     1002    -1     

Flag	Coverage Δ
unittests	87.10% <80.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Webp/WebpDecoderCore.cs	79.89% <0.00%> (ø)
src/ImageSharp/Formats/Png/PngDecoderCore.cs	90.65% <100.00%> (ø)
src/ImageSharp/IO/ChunkedMemoryStream.cs	81.46% <100.00%> (ø)
...ng/Processors/Transforms/Resize/ResizeKernelMap.cs	94.73% <100.00%> (ø)
...rc/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs	97.39% <0.00%> (-0.13%)	⬇️
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	89.22% <0.00%> (+0.18%)	⬆️
...mageSharp/Formats/Webp/Lossless/NearLosslessEnc.cs	100.00% <0.00%> (+7.54%)	⬆️

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