(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/fromKoSave.ts

@@ -0,0 +1,32 @@
+/**
+ * Replicates some of grainjs's fromKo, except that the returned observables have a set() method
+ * which calls koObs.saveOnly(val) rather than koObs(val).
+ */
+import {IKnockoutObservable, KoWrapObs, Observable} from 'grainjs';
+
+const wrappers: WeakMap<IKnockoutObservable<any>, Observable<any>> = new WeakMap();
+
+/**
+ * Returns a Grain.js observable which mirrors a Knockout observable.
+ *
+ * Do not dispose this wrapper, as it is shared by all code using koObs, and its lifetime is tied
+ * to the lifetime of koObs. If unused, it consumes minimal resources, and should get garbage
+ * collected along with koObs.
+ */
+export function fromKoSave<T>(koObs: IKnockoutObservable<T>): Observable<T> {
+  return wrappers.get(koObs) || wrappers.set(koObs, new KoSaveWrapObs(koObs)).get(koObs)!;
+}
+
+export class KoSaveWrapObs<T> extends KoWrapObs<T> {
+  constructor(_koObs: IKnockoutObservable<T>) {
+    if (!('saveOnly' in _koObs)) {
+      throw new Error('fromKoSave needs a saveable observable');
+    }
+    super(_koObs);
+  }
+
+  public set(value: T): void {
+    // Hacky cast to get a private member. TODO: should make it protected instead.
+    (this as any)._koObs.saveOnly(value);
+  }
+}