perf(nodejs): introduce pool into default rng (#513)

* perf(nodejs): introduce pool into default rng

* fixup! address review comments

* fixup! address review comments

Author: puzpuzpuz

src/rng.js

@@ -1,7 +1,12 @@
 import crypto from 'crypto';
 
-const rnds8 = new Uint8Array(16);
+const rnds8Pool = new Uint8Array(256); // # of random values to pre-allocate
+let poolPtr = rnds8Pool.length;
 
 export default function rng() {
-  return crypto.randomFillSync(rnds8);
+  if (poolPtr > rnds8Pool.length - 16) {
+    crypto.randomFillSync(rnds8Pool);
+    poolPtr = 0;
+  }
+  return rnds8Pool.slice(poolPtr, (poolPtr += 16));
 }