Merge pull request #522 from alrun3/unsafe-as

feat: add `Unsafe.As<T>`

Author: yanghuan

CSharp.lua/System.xml

@@ -846,6 +846,10 @@ limitations under the License.
         <method name="op_Inequality" Template="{0} ~= {1}" />
       </class>
     </namespace>
-    <namespace name="System.Runtime.CompilerServices" Name="System" />
+    <namespace name="System.Runtime.CompilerServices" Name="System">
+      <class name="Unsafe">
+        <method name="As" Template="{0}" />
+      </class>
+    </namespace>
 </assembly>
 </meta>