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.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