Fix horrendously bad bit hashing function.

hashBits() is used to generate a hash code from the same 64 bits used
to represent a Wren number as a double. When building a map containing
a large number of integer keys, it's important for this to do a good
job scattering the bits across the 32-bit key space.

Alas, it does not. Worse, the benchmark to test this happens to stop
just before the performance falls off a cliff, so this was easy to
overlook.

This replaces it with the hash function V8 uses, which has much better
performance across the numeric range.

test/benchmark/map_numeric.dart.skip

@@ -6,17 +6,17 @@ main() {
 
   var map = {};
 
-  for (var i = 1; i <= 1000000; i++) {
+  for (var i = 1; i <= 2000000; i++) {
     map[i] = i;
   }
 
   var sum = 0;
-  for (var i = 1; i <= 1000000; i++) {
+  for (var i = 1; i <= 2000000; i++) {
     sum += map[i];
   }
   print(sum);
 
-  for (var i = 1; i <= 1000000; i++) {
+  for (var i = 1; i <= 2000000; i++) {
     map.remove(i);
   }
 

test/benchmark/map_numeric.lua

@@ -2,17 +2,17 @@ local start = os.clock()
 
 local map = {}
 
-for i = 1, 1000000 do
+for i = 1, 2000000 do
   map[i] = i
 end
 
 local sum = 0
-for i = 1, 1000000 do
+for i = 1, 2000000 do
   sum = sum + map[i]
 end
 io.write(string.format("%d\n", sum))
 
-for i = 1, 1000000 do
+for i = 1, 2000000 do
   map[i] = nil
 end
 

test/benchmark/map_numeric.py

@@ -6,15 +6,15 @@ start = time.clock()
 
 map = {}
 
-for i in range(1, 1000001):
+for i in range(1, 2000001):
   map[i] = i
 
 sum = 0
-for i in range(1, 1000001):
+for i in range(1, 2000001):
   sum = sum + map[i]
 print(sum)
 
-for i in range(1, 1000001):
+for i in range(1, 2000001):
   del map[i]
 
 print("elapsed: " + str(time.clock() - start))

test/benchmark/map_numeric.rb

@@ -2,17 +2,17 @@ start = Time.now
 
 map = Hash.new
 
-for i in (1..1000000)
+for i in (1..2000000)
   map[i] = i
 end
 
 sum = 0
-for i in (1..1000000)
+for i in (1..2000000)
   sum = sum + map[i]
 end
 puts sum
 
-for i in (1..1000000)
+for i in (1..2000000)
   map.delete(i)
 end
 

test/benchmark/map_numeric.wren

@@ -2,17 +2,17 @@ var start = System.clock
 
 var map = {}
 
-for (i in 1..1000000) {
+for (i in 1..2000000) {
   map[i] = i
 }
 
 var sum = 0
-for (i in 1..1000000) {
+for (i in 1..2000000) {
   sum = sum + map[i]
 }
 System.print(sum)
 
-for (i in 1..1000000) {
+for (i in 1..2000000) {
   map.remove(i)
 }