Clean Up Namespaces and Visibility

[JimBobSquarePants]

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

Spotted some issues while working on the API docs.

• Made DeflateStrategy and DeflaterEnum internal
• Moved GeometryUtilities to SixLabors.ImageSharp namespace.
• Removed Implementation from SixLabors.ImageSharp.ColorSpaces.Conversion.Implementation

[codecov]

Codecov Report

Merging #1161 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1161   +/-   ##
=======================================
  Coverage   82.27%   82.27%           
=======================================
  Files         681      681           
  Lines       29347    29347           
  Branches     3297     3297           
=======================================
  Hits        24145    24145           
  Misses       4514     4514           
  Partials      688      688           

Flag	Coverage Δ
#unittests	82.27% <ø> (ø)

Impacted Files	Coverage Δ
...c/ImageSharp/ColorSpaces/Companding/LCompanding.cs	0.00% <ø> (ø)
...olorSpaces/Conversion/ColorSpaceConverter.Adapt.cs	68.57% <ø> (ø)
...lorSpaces/Conversion/ColorSpaceConverter.CieLab.cs	100.00% <ø> (ø)
...lorSpaces/Conversion/ColorSpaceConverter.CieLch.cs	100.00% <ø> (ø)
...rSpaces/Conversion/ColorSpaceConverter.CieLchuv.cs	38.88% <ø> (ø)
...lorSpaces/Conversion/ColorSpaceConverter.CieLuv.cs	100.00% <ø> (ø)
...lorSpaces/Conversion/ColorSpaceConverter.CieXyy.cs	92.30% <ø> (ø)
...lorSpaces/Conversion/ColorSpaceConverter.CieXyz.cs	93.95% <ø> (ø)
...ColorSpaces/Conversion/ColorSpaceConverter.Cmyk.cs	84.61% <ø> (ø)
.../ColorSpaces/Conversion/ColorSpaceConverter.Hsl.cs	53.84% <ø> (ø)
... and 44 more

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[antonfirsov]

Nice, LGTM!