runtime: rewrite lots of foo_Bar(f, ...) into f.bar(...)

Applies to types fixAlloc, mCache, mCentral, mHeap, mSpan, and
mSpanList.

Two special cases:

1. mHeap_Scavenge() previously didn't take an *mheap parameter, so it
was specially handled in this CL.

2. mHeap_Free() would have collided with mheap's "free" field, so it's
been renamed to (*mheap).freeSpan to parallel its underlying
(*mheap).freeSpanLocked method.

Change-Id: I325938554cca432c166fe9d9d689af2bbd68de4b
Reviewed-on: https://go-review.googlesource.com/16221


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>