fix: support webworkers

Edge does not appear to make a global `crypto` variable available in
WebWorkers however the WebCrypto seems to be exposed on the `self`
variable, see sodium-friends/sodium-javascript#8

Since window.self also points to the window object in browsers checking
self should work both, in the browser as well as in WebWorkers.

Fixes #392

src/rng-browser.js

@@ -4,11 +4,16 @@
 
 // getRandomValues needs to be invoked in a context where "this" is a Crypto implementation. Also,
 // find the complete implementation of crypto (msCrypto) on IE11.
+// Use self in order to support WebWorkers as well, see
+// https://developer.mozilla.org/en-US/docs/Web/API/Window/self
 var getRandomValues =
-  (typeof crypto != 'undefined' && crypto.getRandomValues && crypto.getRandomValues.bind(crypto)) ||
-  (typeof msCrypto != 'undefined' &&
-    typeof window.msCrypto.getRandomValues == 'function' &&
-    msCrypto.getRandomValues.bind(msCrypto));
+  typeof self !== 'undefined' &&
+  ((typeof self.crypto !== 'undefined' &&
+    self.crypto.getRandomValues &&
+    self.crypto.getRandomValues.bind(self.crypto)) ||
+    (typeof self.msCrypto !== 'undefined' &&
+      self.msCrypto.getRandomValues &&
+      self.msCrypto.getRandomValues.bind(self.msCrypto)));
 
 var rnds8 = new Uint8Array(16); // eslint-disable-line no-undef
 export default function rng() {