Combine NoScan allocations < 16 bytes into a single memory block.
Reduces number of allocations on json/garbage benchmarks by 10+%.

json-1
allocated                 8039872      7949194      -1.13%
allocs                     105774        93776     -11.34%
cputime                 156200000    100700000     -35.53%
gc-pause-one              4908873      3814853     -22.29%
gc-pause-total            2748969      2899288      +5.47%
rss                      52674560     43560960     -17.30%
sys-gc                    3796976      3256304     -14.24%
sys-heap                 43843584     35192832     -19.73%
sys-other                 5589312      5310784      -4.98%
sys-stack                  393216       393216      +0.00%
sys-total                53623088     44153136     -17.66%
time                    156193436    100886714     -35.41%
virtual-mem             256548864    256540672      -0.00%

garbage-1
allocated                 2996885      2932982      -2.13%
allocs                      62904        55200     -12.25%
cputime                  17470000     17400000      -0.40%
gc-pause-one            932757485    925806143      -0.75%
gc-pause-total            4663787      4629030      -0.75%
rss                    1151074304   1133670400      -1.51%
sys-gc                   66068352     65085312      -1.49%
sys-heap               1039728640   1024065536      -1.51%
sys-other                38038208     37485248      -1.45%
sys-stack                 8650752      8781824      +1.52%
sys-total              1152485952   1135417920      -1.48%
time                     17478088     17418005      -0.34%
virtual-mem            1343709184   1324204032      -1.45%

LGTM=iant, bradfitz
R=golang-codereviews, dave, iant, rsc, bradfitz
CC=golang-codereviews, khr
https://golang.org/cl/38750047