[js] Standardize the unboxing of too big bigints error

The size is likely misleading and should be changed on both this and moar backends

Author: pmurias

src/vm/js/nqp-runtime/bignum.js

@@ -304,13 +304,17 @@ op.radix_I = function(currentHLL, radix, str, zpos, flags, type) {
 function bitSize(n) {
   let bits = 0;
 
+  if (n < 0) n = -n;
+
   while (n) {
     n = n >> 1n;
     bits++;
   }
   return bits;
 }
 
+exports.bitSize = bitSize;
+
 function randomWithSameBitSize(n) {
   let got = 0n;
   let needed = bitSize(n);

src/vm/js/nqp-runtime/reprs.js

@@ -46,6 +46,8 @@ const EDGE_CODEPOINT_I_LL = 15;
 const reprs = {};
 const reprById = [];
 
+const bignum = require('./bignum.js');
+
 function basicTypeObjectFor(HOW) {
   const st = new sixmodel.STable(this, HOW);
   this._STable = st;
@@ -1597,12 +1599,12 @@ function getBI(obj) {
 }
 
 
-function getIntFromBI(bignum) {
-  if (bignum < -(2n**64n) || 2n**64n <= bignum) {
+function getIntFromBI(n) {
+  if (n < -(2n**63n) || 2n**63n <= n) {
     // TODO - put exact number of bits into exception
-    throw new NQPException(`Cannot unbox too big bigint into native integer`);
+    throw new NQPException(`Cannot unbox ${bignum.bitSize(n)} bit wide bigint into native integer`);
   } else {
-    return Number(bignum) | 0;
+    return Number(n) | 0;
   }
 }