MCaches	now hold a MSpan for each sizeclass which they have
exclusive access to allocate from, so no lock is needed.

Modifying the heap bitmaps also no longer requires a cas.

runtime.free gets more expensive.  But we don't use it
much any more.

It's not much faster on 1 processor, but it's a lot
faster on multiple processors.

benchmark                 old ns/op    new ns/op    delta
BenchmarkSetTypeNoPtr1           24           23   -0.42%
BenchmarkSetTypeNoPtr2           33           34   +0.89%
BenchmarkSetTypePtr1             51           49   -3.72%
BenchmarkSetTypePtr2             55           54   -1.98%

benchmark                old ns/op    new ns/op    delta
BenchmarkAllocation          52739        50770   -3.73%
BenchmarkAllocation-2        33957        34141   +0.54%
BenchmarkAllocation-3        33326        29015  -12.94%
BenchmarkAllocation-4        38105        25795  -32.31%
BenchmarkAllocation-5        68055        24409  -64.13%
BenchmarkAllocation-6        71544        23488  -67.17%
BenchmarkAllocation-7        68374        23041  -66.30%
BenchmarkAllocation-8        70117        20758  -70.40%

LGTM=rsc, dvyukov
R=dvyukov, bradfitz, khr, rsc
CC=golang-codereviews
https://golang.org/cl/46810043