runtime: rename mheap.maps to mheap.spans
as was dicussed in cl/9791044

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/9853046
Merge branch 'master' into gitfix
testing: quantize AllocsPerRun
As the code now says:

We are forced to return a float64 because the API is silly, but do
the division as integers so we can ask if AllocsPerRun()==1
instead of AllocsPerRun()<2.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/9837049
doc/go1.2.txt: AllocsPerRun now quantized

R=golang-dev, khr, rsc
CC=golang-dev
https://codereview.appspot.com/9728045
runtime: set MSpan.limit properly for large spans.
Then use the limit to make sure MHeap_LookupMaybe & inlined
copies don't return a span if the pointer is beyond the limit.
Use this fact to optimize all call sites.

R=golang-dev, dvyukov
CC=golang-dev
https://codereview.appspot.com/9869045
runtime: introduce helper persistentalloc() function
It is a caching wrapper around SysAlloc() that can allocate small chunks.
Use it for symtab allocations. Reduces number of symtab walks from 4 to 3
(reduces buildfuncs time from 10ms to 7.5ms on a large binary,
reduces initial heap size by 680K on the same binary).
Also can be used for type info allocation, itab allocation.
There are also several places in GC where we do the same thing,
they can be changed to use persistentalloc().
Also can be used in FixAlloc, because each instance of FixAlloc allocates
in 128K regions, which is too eager.
Reincarnation of committed and rolled back https://codereview.appspot.com/9805043
The latent bugs that it revealed are fixed:
https://codereview.appspot.com/9837049
https://codereview.appspot.com/9778048

R=golang-dev, khr
CC=golang-dev
https://codereview.appspot.com/9778049
runtime: fix heap coalescing bug introduced in cl/9802043
mheap.map become a pointer, so nelem(h->map) returns 1 rather than the map size.
As the result coalescing with subsequent spans does not happen.

R=golang-dev, khr
CC=golang-dev
https://codereview.appspot.com/9649046
Merge branch 'master' into gitfix