Move MemoryStream to shared CoreLib partition (#15116)

- Apply formating and other minor changes from CoreRT
    - Prefer async implementations from CoreCLR since they are more recent and better optimized
- Apply similar changes to UnmanagedMemoryStream as well to keep the two close

Author: jkotas

src/mscorlib/System.Private.CoreLib.csproj

@@ -518,7 +518,6 @@
     <Compile Include="$(BclSourcesRoot)\System\IO\File.cs" />
     <Compile Include="$(BclSourcesRoot)\System\IO\FileLoadException.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\IO\FileNotFoundException.CoreCLR.cs" />
-    <Compile Include="$(BclSourcesRoot)\System\IO\MemoryStream.cs" />
     <Compile Include="$(BclSourcesRoot)\System\IO\Stream.cs" />
     <Compile Include="$(BclSourcesRoot)\System\IO\UnmanagedMemoryAccessor.cs" />
   </ItemGroup>

src/mscorlib/shared/System.Private.CoreLib.Shared.projitems

@@ -199,6 +199,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileShare.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\FileStream.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\IOException.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)System\IO\MemoryStream.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\Path.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PathInternal.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\IO\PathTooLongException.cs" />