Random hashes with a more cache-friendly distribution

Random hashes with a more cache-friendly distribution

This is a hashcode implementation for ES6 Map and Set that gives better locality of reference.  This is a different more complicated version of https://codereview.chromium.org/1157073002/ that is less likely to cause
problems for an open addressing hash (but note that ES6 Map and Set do not currently use open addressing).  I put it here for reference.

This one also gives a good speedup on the modified example from bug 4086.  Where the original CL lowers runtime by 62%, this change only gives about 53% reduction.

R=adamk@chromium.org, verwaest@chromium.org
BUG=

[Erik Corry Chromium.org, 2015-05-26 21:56:56]

https://codereview.chromium.org/1158083002/diff/20001/src/math.js
File src/math.js (right):

https://codereview.chromium.org/1158083002/diff/20001/src/math.js#newcode160
src/math.js:160: level_0 += 4;
This should perhaps be co-prime with powers of 2.  5 seems to work well too, for
example.