Add monitor node for graph introspection

Adds a `serialize` function on Node which can be implemented by nodes to
allow introspecting their content. This pr also adds a Monitor Node that
always caches the last value it was evaluated with.

Test Plan: -

Reviewers: 0HyperCube

Reviewed By: 0HyperCube

Pull Request: https://github.com/GraphiteEditor/Graphite/pull/1165

node-graph/gcore/src/lib.rs

@@ -37,6 +37,10 @@ pub trait Node<'i, Input: 'i>: 'i {
 	type Output: 'i;
 	fn eval(&'i self, input: Input) -> Self::Output;
 	fn reset(self: Pin<&mut Self>) {}
+	#[cfg(feature = "alloc")]
+	fn serialize(&self) -> Option<String> {
+		None
+	}
 }
 
 #[cfg(feature = "alloc")]