Revert "bufferutils: remove equal, use Buffer.compare"

This reverts commit 75540b6116.
See discussion in https://github.com/bitcoinjs/bitcoinjs-lib/pull/410

src/bufferutils.js

@@ -168,6 +168,16 @@ function varIntBuffer (i) {
   return buffer
 }
 
+function equal (a, b) {
+  if (a.length !== b.length) return false
+
+  for (var i = 0; i < a.length; ++i) {
+    if (a[i] !== b[i]) return false
+  }
+
+  return true
+}
+
 function reverse (buffer) {
   var buffer2 = new Buffer(buffer)
   Array.prototype.reverse.call(buffer2)
@@ -175,6 +185,7 @@ function reverse (buffer) {
 }
 
 module.exports = {
+  equal: equal,
   pushDataSize: pushDataSize,
   readPushDataInt: readPushDataInt,
   readUInt64LE: readUInt64LE,