Rename Number.prototype.clz() to Math.clz32().

This was decided in the Jan 28, 2014 ES6 meeting:
https://github.com/rwaldron/tc39-notes/blob/master/es6/2014-01/jan-28.md#clz-count-leading-zeros

There are some questions WRT the spec:
https://bugs.ecmascript.org/show_bug.cgi?id=2465