Adjust preprocessors (#44)

This picks the best overloads for more TFMs.

And uses `string.Contains(string)` instead of
`IEnumerable<char>.Contains(char)` for older TFMs.

src/Reflectify/Reflectify.cs

@@ -192,7 +192,7 @@ public static bool IsTuple(this Type type)
             return false;
         }
 
-#if !(NET47 || NETSTANDARD2_0)
+#if NETCOREAPP2_0_OR_GREATER || NET471_OR_GREATER || NETSTANDARD2_1_OR_GREATER
         return typeof(ITuple).IsAssignableFrom(type);
 #else
         Type openType = type.GetGenericTypeDefinition();
@@ -524,10 +524,10 @@ public static bool IsIndexer(this PropertyInfo member)
     /// </summary>
     public static bool IsExplicitlyImplemented(this PropertyInfo prop)
     {
-#if NETCOREAPP3_0_OR_GREATER
-        return prop.Name.Contains('.', StringComparison.OrdinalIgnoreCase);
-#else
+#if NETCOREAPP2_1_OR_GREATER || NETSTANDARD2_1_OR_GREATER
         return prop.Name.Contains('.');
+#else
+        return prop.Name.IndexOf('.') != -1;
 #endif
     }