Preserve redo log for mutable objects

Replay it during time travel debugging

src/runtime/let_multiversion.js

@@ -0,0 +1,61 @@
+import {Multiversion} from './multiversion.js'
+
+// https://stackoverflow.com/a/29018745
+function binarySearch(arr, el, compare_fn) {
+    let m = 0;
+    let n = arr.length - 1;
+    while (m <= n) {
+        let k = (n + m) >> 1;
+        let cmp = compare_fn(el, arr[k]);
+        if (cmp > 0) {
+            m = k + 1;
+        } else if(cmp < 0) {
+            n = k - 1;
+        } else {
+            return k;
+        }
+    }
+    return ~m;
+}
+
+export class LetMultiversion extends Multiversion {
+  constructor(cxt, initial) {
+    super(cxt)
+    this.latest = initial
+    this.versions = [{version_number: cxt.version_counter, value: initial}]
+  }
+
+  rollback_if_needed() {
+    if(this.needs_rollback()) {
+      this.latest = this.get_version(this.cxt.version_counter)
+    }
+  }
+
+  get() {
+    this.rollback_if_needed()
+    return this.latest
+  }
+
+  set(value) {
+    this.rollback_if_needed()
+    const version_number = ++this.cxt.version_counter
+    if(this.is_created_during_current_expansion()) {
+      this.versions.push({version_number, value})
+    }
+    this.latest = value
+  }
+
+  get_version(version_number) {
+    if(version_number == null) {
+      throw new Error('illegal state')
+    }
+    const idx = binarySearch(this.versions, version_number, (id, el) => id - el.version_number)
+    if(idx >= 0) {
+      return this.versions[idx].value
+    } else if(idx == -1) {
+      throw new Error('illegal state')
+    } else {
+      return this.versions[-idx - 2].value
+    }
+  }
+}