Move libraries to .NET Standard, allowing Xamarin.macOS GUI to compile.

DiscImageChef.Checksums.csproj

@@ -10,7 +10,6 @@
     <AssemblyName>DiscImageChef.Checksums</AssemblyName>
     <ReleaseVersion>$(Version)</ReleaseVersion>
     <EnableDefaultCompileItems>false</EnableDefaultCompileItems>
-    <TargetFramework>net461</TargetFramework>
     <GenerateAssemblyInfo>true</GenerateAssemblyInfo>
     <Version>4.5.99.1693</Version>
     <Company>Claunia.com</Company>
@@ -18,6 +17,7 @@
     <Product>The Disc Image Chef</Product>
     <Title>DiscImageChef.Checksums</Title>
     <ApplicationVersion>$(Version)</ApplicationVersion>
+    <TargetFrameworks>net461;netstandard2.0</TargetFrameworks>
   </PropertyGroup>
   <PropertyGroup>
     <NrtRevisionFormat>$(Version)-{chash:8} built by {mname} in $(Configuration){!:, modified}</NrtRevisionFormat>

RIPEMD160Context.cs

@@ -30,6 +30,8 @@
 // Copyright © 2011-2018 Natalia Portillo
 // ****************************************************************************/
 
+#if !NETSTANDARD2_0
+
 using System.IO;
 using System.Security.Cryptography;
 using System.Text;
@@ -152,4 +154,6 @@ namespace DiscImageChef.Checksums
             return Data(data, (uint)data.Length, out hash);
         }
     }
-}
+}
+
+#endif