Use BN.red in modexp precompile

Add checks for E == 0 and M == 1

Fix M == 1 check

lib/precompiled/05-modexp.js

@@ -56,14 +56,14 @@ function getAdjustedExponentLength (data) {
   return adjustedExpLen
 }
 
-// Taken from https://stackoverflow.com/a/1503019
 function expmod (B, E, M) {
   if (E.isZero()) return new BN(1).mod(M)
-  var BM = B.mod(M)
-  var R = expmod(BM, E.divn(2), M)
-  R = (R.mul(R)).mod(M)
-  if (E.mod(new BN(2)).isZero()) return R
-  return (R.mul(BM)).mod(M)
+  // Red asserts M > 1
+  if (M.cmpn(1) === 0) return new BN(0)
+  const red = BN.red(M)
+  const redB = B.toRed(red)
+  const res = redB.redPow(E)
+  return res.fromRed()
 }
 
 function getOOGResults (opts, results) {