Move Span APIs that allow skipping visibility checks to MemoryMarshal

[ahsonkhan]

Resolves https://github.com/dotnet/corefx/issues/27094

TODO: Remove AsBytes extension method once all calls have been changed to MemoryMarshal

Depends on the PR from coreclr (along with the mirror PR) - dotnet/coreclr#17087 / dotnet/coreclr#17106. It will fail CI until then.

cc @jkotas, @KrzysztofCwalina, @dotnet/corefxlab-contrib

[jkotas]

@dotnet-bot test this please

[joshfree]

09:05:16 Build FAILED.
09:05:16 
09:05:16 Perf.Span.BinaryReadAndWrite.cs(29,38): error CS0103: The name 'ReadMachineEndian' does not exist in the current context [D:\j\workspace\windows-TGrou---74aa877a\src\System.Memory\tests\Performance\System.Memory.Performance.Tests.csproj]
09:05:16 Perf.Span.BinaryReadAndWrite.cs(64,38): error CS0103: The name 'ReadMachineEndian' does not exist in the current context [D:\j\workspace\windows-TGrou---74aa877a\src\System.Memory\tests\Performance\System.Memory.Performance.Tests.csproj]
09:05:16     0 Warning(s)
09:05:16     2 Error(s)
09:05:16 
09:05:16 Time Elapsed 00:01:16.06
09:05:16 Command execution failed with exit code 1.

[stephentoub]

@dotnet-bot test this please

[ahsonkhan]

Closing this PR as the commits got cherry-picked into the mirror PR - #28384