(core) move client code to core

Summary:
This moves all client code to core, and makes minimal fix-ups to
get grist and grist-core to compile correctly.  The client works
in core, but I'm leaving clean-up around the build and bundles to
follow-up.

Test Plan: existing tests pass; server-dev bundle looks sane

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2627

app/client/lib/koArrayWrap.ts

@@ -0,0 +1,42 @@
+import {KoArray} from 'app/client/lib/koArray';
+import {IDisposableOwnerT, MutableObsArray, ObsArray, setDisposeOwner} from 'grainjs';
+
+/**
+ * Returns a grainjs ObsArray that reflects the given koArray, mapping small changes using
+ * similarly efficient events.
+ *
+ * (Note that for both ObsArray and koArray, the main purpose in life is to be more efficient than
+ * an array-valued observable by handling small changes more efficiently.)
+ */
+export function createObsArray<T>(
+  owner: IDisposableOwnerT<ObsArray<T>> | null,
+  koArray: KoArray<T>,
+): ObsArray<T> {
+  return setDisposeOwner(owner, new KoWrapObsArray(koArray));
+}
+
+
+/**
+ * An Observable that wraps a Knockout observable, created via fromKo(). It keeps minimal overhead
+ * when unused by only subscribing to the wrapped observable while it itself has subscriptions.
+ *
+ * This way, when unused, the only reference is from the wrapper to the wrapped object. KoWrapObs
+ * should not be disposed; its lifetime is tied to that of the wrapped object.
+ */
+class KoWrapObsArray<T> extends MutableObsArray<T> {
+  private _koSub: any = null;
+
+  constructor(_koArray: KoArray<T>) {
+    super(Array.from(_koArray.peek()));
+
+    this._koSub = _koArray.subscribe((splice: any) => {
+      const newValues = splice.array.slice(splice.start, splice.start + splice.added);
+      this.splice(splice.start, splice.deleted.length, ...newValues);
+    }, null, 'spliceChange');
+  }
+
+  public dispose(): void {
+    this._koSub.dispose();
+    super.dispose();
+  }
+}