(core) move home server into core

Summary: This moves enough server material into core to run a home server.  The data engine is not yet incorporated (though in manual testing it works when ported).

Test Plan: existing tests pass

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2552

app/common/arrayToString.ts

@@ -0,0 +1,28 @@
+/**
+ * Functions to convert between an array of bytes and a string. The implementations are
+ * different for Node and for the browser.
+ */
+
+declare const TextDecoder: any, TextEncoder: any;
+
+export let arrayToString: (data: Uint8Array) => string;
+export let stringToArray: (data: string) => Uint8Array;
+
+if (typeof TextDecoder !== 'undefined') {
+  // Note that constructing a TextEncoder/Decoder takes time, so it's faster to reuse.
+  const dec = new TextDecoder('utf8');
+  const enc = new TextEncoder('utf8');
+  arrayToString = function(uint8Array: Uint8Array): string {
+    return dec.decode(uint8Array);
+  };
+  stringToArray = function(str: string): Uint8Array {
+    return enc.encode(str);
+  };
+} else {
+  arrayToString = function(uint8Array: Uint8Array): string {
+    return Buffer.from(uint8Array).toString('utf8');
+  };
+  stringToArray = function(str: string): Uint8Array {
+    return new Uint8Array(Buffer.from(str, 'utf8'));
+  };
+}