add a lot more logging to both client and server to analyze #81

> #81 Compiling brush doesn't seem to finish correctly sometimes, which causes loss of data
> Sometimes it seems like the client state can get desynced (server render thread dies due to a panic?) and then the server starts dropping all requests for drawing the brush.
> These panics should never happen of course, but we need better logging first to determine the exact cause.

crates/haku/src/ast.rs

@@ -74,6 +74,14 @@ impl Ast {
         ast
     }
 
+    pub fn len(&self) -> usize {
+        self.kinds.len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
     pub fn alloc(&mut self, kind: NodeKind, span: Span) -> Result<NodeId, NodeAllocError> {
         if self.kinds.len() >= self.kinds.capacity() {
             return Err(NodeAllocError);