Temporary workaround for BN#_move (#236)

Solve issues for using with old bn.js versions (<5.0.0).
See details at: indutny/elliptic#191

lib/bn.js

@@ -327,11 +327,15 @@
     dest.red = this.red;
   };
 
+  function move (dest, src) {
+    dest.words = src.words;
+    dest.length = src.length;
+    dest.negative = src.negative;
+    dest.red = src.red;
+  }
+
   BN.prototype._move = function _move (dest) {
-    dest.words = this.words;
-    dest.length = this.length;
-    dest.negative = this.negative;
-    dest.red = this.red;
+    move(dest, this);
   };
 
   BN.prototype.clone = function clone () {
@@ -3243,7 +3247,7 @@
   Red.prototype.imod = function imod (a) {
     if (this.prime) return this.prime.ireduce(a)._forceRed(this);
 
-    a.umod(this.m)._forceRed(this)._move(a);
+    move(a, a.umod(this.m)._forceRed(this));
     return a;
   };