This is meant as a replacement for the Rotate Euler node. Overall it
should be more performant, and the clarity given by the separate socket
type should help distinguish its purpose.

The Rotate Euler node is removed from search and the add menu by this
commit. In the future it can be versioned away. That isn't done now to
avoid a breaking API change and to make this commit less risky.

@@ -0,0 +12,4 @@
+{
+  b.is_function_node();
+  b.add_input<decl::Rotation>("Rotation 1");
+  b.add_input<decl::Rotation>("Rotation 2");