separate out strudel.mjs, make index.mjs aggregate module

packages/core/state.mjs

@@ -0,0 +1,22 @@
+export class State {
+  constructor(span, controls = {}) {
+    this.span = span;
+    this.controls = controls;
+  }
+
+  // Returns new State with different span
+  setSpan(span) {
+    return new State(span, this.controls);
+  }
+
+  withSpan(func) {
+    return this.setSpan(func(this.span));
+  }
+
+  // Returns new State with different controls
+  setControls(controls) {
+    return new State(this.span, controls);
+  }
+}
+
+export default State;