[js] Use naive directly from the spec asIntN and asUIntN

That avoid using BigInt and will be replaced once efficent versions are in jsbi

Author: pmurias

package.json

@@ -12,6 +12,7 @@
     "eslint-config-google": "^0.8.0"
   },
   "dependencies": {
+    "jsbi": "^2.0.2",
     "nqp-runtime": "file:src/vm/js/nqp-runtime"
   }
 }

src/vm/js/nqp-runtime/as-int-n.js

@@ -1,10 +1,12 @@
 const JSBI = require('jsbi');
 
 module.exports.asIntN = function(bits, bigint) {
-  return JSBI.BigInt(BigInt.asIntN(bits, BigInt(bigint.toString())).toString());
+  const power = JSBI.exponentiate(JSBI.BigInt(2), JSBI.BigInt(bits));
+  const mod = JSBI.remainder(bigint, power);
+  return (JSBI.greaterThanOrEqual(mod, JSBI.exponentiate(JSBI.BigInt(2), JSBI.BigInt(bits-1))) ? JSBI.subtract(mod, power) : mod);
 };
 
 module.exports.asUintN = function(bits, bigint) {
-  return JSBI.BigInt(BigInt.asUintN(bits, BigInt(bigint.toString())).toString());
+  return JSBI.remainder(bigint, JSBI.exponentiate(JSBI.BigInt(2), JSBI.BigInt(bits)));
 };