feat: get full evaluation back online

Very hacky, but this is enough to be finished with the graph IR for now.

crates/eval/src/value/mod.rs

@@ -1,5 +1,12 @@
 use image::DynamicImage;
 
-pub enum Dynamic {
+/// Any runtime value that an instruction can input or output.
+///
+/// The name is taken from [Godot's `Variant` type],
+/// which is very similar to this one.
+///
+/// [Godot's `Variant` type]: https://docs.godotengine.org/en/stable/classes/class_variant.html
+#[derive(Clone, Debug)]
+pub enum Variant {
     Image(DynamicImage),
 }