[IMP] reactivity: overhaul reactivity system

This commit makes the reactivity system more fine grained and makes it
more eager to stop observing keys or objects when they are modified,
this results in fewer "false positive" notifications.

tests/__snapshots__/reactivity.test.ts.snap

@@ -1,5 +1,51 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`Reactivity: useState concurrent renderings 1`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component } = bdom;
+  let { withDefault, getTemplate, prepareList, withKey, zero, call, callSlot, capture, isBoundary, shallowEqual, setContextValue, toNumber, safeOutput } = helpers;
+  
+  let block1 = createBlock(\`<span><block-text-0/><block-text-1/></span>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let d1 = ctx['context'][ctx['props'].key].n;
+    let d2 = ctx['state'].x;
+    return block1([d1, d2]);
+  }
+}"
+`;
+
+exports[`Reactivity: useState concurrent renderings 2`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component } = bdom;
+  let { withDefault, getTemplate, prepareList, withKey, zero, call, callSlot, capture, isBoundary, shallowEqual, setContextValue, toNumber, safeOutput } = helpers;
+  
+  let block1 = createBlock(\`<p><block-child-0/></p>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let b2 = component(\`ComponentC\`, {key: ctx['props'].key}, key + \`__1\`, node, ctx);
+    return block1([], [b2]);
+  }
+}"
+`;
+
+exports[`Reactivity: useState concurrent renderings 3`] = `
+"function anonymous(bdom, helpers
+) {
+  let { text, createBlock, list, multi, html, toggler, component } = bdom;
+  let { withDefault, getTemplate, prepareList, withKey, zero, call, callSlot, capture, isBoundary, shallowEqual, setContextValue, toNumber, safeOutput } = helpers;
+  
+  let block1 = createBlock(\`<div><block-child-0/></div>\`);
+  
+  return function template(ctx, node, key = \\"\\") {
+    let b2 = component(\`ComponentB\`, {key: ctx['context'].key}, key + \`__1\`, node, ctx);
+    return block1([], [b2]);
+  }
+}"
+`;
+
 exports[`Reactivity: useState destroyed component before being mounted is inactive 1`] = `
 "function anonymous(bdom, helpers
 ) {