General code clean up of System.Memory

[ahsonkhan]

• Use explicit type instead of var where type can't be inferred easily.
• Removing unnecessary use of 'unsafe' keyword and generic type specifiers.

As an aside, change comment in LinuxNetworkInterface.cs to the right type - OperationalStatus (to avoid confusion with OperationStatus)

cc @KrzysztofCwalina, @jkotas, @stephentoub

[ahsonkhan]

System.IO.Compression.Tests Dispose_WithUnfinishedWriteAsync test failed on Windows.7.Amd64.Open
https://mc.dot.net/#/user/ahsonkhan/pr~2Fjenkins~2Fdotnet~2Fcorefx~2Fmaster~2F/test~2Ffunctional~2Fcli~2F/87216ce10f7b0f5fd51ccdd911896ff3f4e39ccb/workItem/System.IO.Compression.Tests/analysis/xunit/System.IO.Compression.DeflateStreamUnitTests~2FDispose_WithUnfinishedWriteAsync

Assert.Throws() Failure
Expected: typeof(System.AggregateException)
Actual:   (No exception was thrown)
Stack Trace :
   at System.IO.Compression.CompressionStreamUnitTestBase.<Dispose_WithUnfinishedWriteAsync>d__5.MoveNext() in D:\j\workspace\windows-TGrou---f8ac6754\src\Common\tests\System\IO\Compression\CompressionStreamUnitTestBase.cs:line 208
--- End of stack trace from previous location where exception was thrown ---
--- End of stack trace from previous location where exception was thrown ---
--- End of stack trace from previous location where exception was thrown ---

cc @ianhays

System.Net.Ping.Functional.Tests PacketSizeIsRespected(payloadSize: 0) test failed on OSX.1012.Amd64.Open-Debug-x64
https://mc.dot.net/#/user/ahsonkhan/pr~2Fjenkins~2Fdotnet~2Fcorefx~2Fmaster~2F/test~2Ffunctional~2Fcli~2F/87216ce10f7b0f5fd51ccdd911896ff3f4e39ccb/workItem/System.Net.Ping.Functional.Tests/analysis/xunit/System.Net.NetworkInformation.Tests.UnixPingUtilityTests~2FPacketSizeIsRespected(payloadSize:%200)

System.Exception : Ping output was <>
---- System.ArgumentOutOfRangeException : Length cannot be less than zero.
Parameter name: length
Stack Trace :
   at System.Net.NetworkInformation.Tests.UnixPingUtilityTests.<PacketSizeIsRespected>d__1.MoveNext() in /Users/dotnet-bot/j/workspace/dotnet_corefx/master/osx-TGroup_netcoreapp+CGroup_Debug+AGroup_x64+TestOuter_false_prtest/src/System.Net.Ping/tests/FunctionalTests/UnixPingUtilityTests.cs:line 68
--- End of stack trace from previous location where exception was thrown ---
--- End of stack trace from previous location where exception was thrown ---
--- End of stack trace from previous location where exception was thrown ---
----- Inner Stack Trace -----
   at System.String.Substring(Int32 startIndex, Int32 length) in /Users/buildagent/agent/_work/289/s/src/mscorlib/src/System/String.Manipulation.cs:line 1559
   at System.Net.NetworkInformation.Tests.UnixPingUtilityTests.ParseReturnedPacketSize(String pingOutput) in /Users/dotnet-bot/j/workspace/dotnet_corefx/master/osx-TGroup_netcoreapp+CGroup_Debug+AGroup_x64+TestOuter_false_prtest/src/System.Net.Ping/tests/FunctionalTests/UnixPingUtilityTests.cs:line 76
   at System.Net.NetworkInformation.Tests.UnixPingUtilityTests.<PacketSizeIsRespected>d__1.MoveNext() in /Users/dotnet-bot/j/workspace/dotnet_corefx/master/osx-TGroup_netcoreapp+CGroup_Debug+AGroup_x64+TestOuter_false_prtest/src/System.Net.Ping/tests/FunctionalTests/UnixPingUtilityTests.cs:line 55

cc @Priya91

[ahsonkhan]

@dotnet-bot test OSX x64 Debug Build
@dotnet-bot test Windows x86 Release Build