reduce browser bundle size

reduces browser bundle size by ~612k or ~92%.

src/util/browser-support.ts

@@ -0,0 +1,18 @@
+
+export async function signMessage(message: string, secret: string): Promise<string> {
+  const encoder = new TextEncoder();
+  const key = await window.crypto.subtle.importKey(
+    'raw',
+    encoder.encode(secret),
+    {name: 'HMAC', hash: {name: 'SHA-256'}},
+    false,
+    ['sign']
+  );
+
+  const signature = await window.crypto.subtle.sign('HMAC', key, encoder.encode(message));
+
+  return Array.prototype.map.call(
+    new Uint8Array(signature),
+    (x: any) => ('00'+x.toString(16)).slice(-2)
+  ).join('');
+};