der: encode integers > 4294967295 and BigInts

[jablko]

Before

const INTEGER = asn1.define('INTEGER', function() {
  this.int();
});
console.log(INTEGER.decode(INTEGER.encode(0x100000000))); // <BN: 0>
console.log(INTEGER.decode(INTEGER.encode(1n))); // TypeError: num.toArray is not a function

After

console.log(INTEGER.decode(INTEGER.encode(0x100000000))); // <BN: 100000000>
console.log(INTEGER.decode(INTEGER.encode(1n))); // <BN: 1>