style: Move the guts of calc nodes into a generic enum.

We'll have different leaf nodes as we progress in the value computation stage.

Differential Revision: https://phabricator.services.mozilla.com/D63396

components/style/values/generics/mod.rs

@@ -19,6 +19,7 @@ pub mod basic_shape;
 pub mod border;
 #[path = "box.rs"]
 pub mod box_;
+pub mod calc;
 pub mod color;
 pub mod column;
 pub mod counters;