Compiler instrumentation shows that the cap of the stores slice in the
storeOrder function is almost always 64 or less. Since the slice does
not escape, pre-allocating on the stack a 64-elements one greatly
reduces the number of allocations performed by the function.

name      old time/op       new time/op       delta
Template        289ms ± 5%        283ms ± 3%  -1.99%  (p=0.000 n=19+20)
Unicode         140ms ± 6%        136ms ± 6%  -2.61%  (p=0.021 n=19+20)
GoTypes         915ms ± 2%        895ms ± 2%  -2.24%  (p=0.000 n=19+20)
Compiler        4.15s ± 1%        4.04s ± 2%  -2.73%  (p=0.000 n=20+20)
SSA             10.0s ± 1%         9.8s ± 2%  -2.13%  (p=0.000 n=20+20)
Flate           189ms ± 6%        186ms ± 4%  -1.75%  (p=0.028 n=19+20)
GoParser        229ms ± 5%        224ms ± 4%  -2.25%  (p=0.001 n=20+19)
Reflect         584ms ± 2%        573ms ± 3%  -1.83%  (p=0.000 n=18+20)
Tar             265ms ± 3%        261ms ± 3%  -1.33%  (p=0.021 n=20+20)
XML             328ms ± 2%        321ms ± 2%  -2.11%  (p=0.000 n=20+20)

name      old user-time/op  new user-time/op  delta
Template        408ms ± 4%        400ms ± 4%  -1.98%  (p=0.006 n=19+20)
Unicode         216ms ± 9%        216ms ± 7%    ~     (p=0.883 n=20+20)
GoTypes         1.25s ± 1%        1.23s ± 3%  -1.32%  (p=0.002 n=19+20)
Compiler        5.77s ± 1%        5.69s ± 2%  -1.47%  (p=0.000 n=18+19)
SSA             14.6s ± 5%        14.1s ± 4%  -3.45%  (p=0.000 n=20+20)
Flate           252ms ± 7%        251ms ± 7%    ~     (p=0.659 n=20+20)
GoParser        314ms ± 5%        310ms ± 5%    ~     (p=0.165 n=20+20)
Reflect         780ms ± 2%        769ms ± 3%  -1.34%  (p=0.004 n=19+18)
Tar             365ms ± 7%        367ms ± 5%    ~     (p=0.841 n=20+20)
XML             439ms ± 4%        432ms ± 4%  -1.45%  (p=0.043 n=20+20)

name      old alloc/op      new alloc/op      delta
Template       38.9MB ± 0%       38.8MB ± 0%  -0.26%  (p=0.000 n=19+20)
Unicode        29.0MB ± 0%       29.0MB ± 0%  -0.02%  (p=0.001 n=20+19)
GoTypes         115MB ± 0%        115MB ± 0%  -0.31%  (p=0.000 n=20+20)
Compiler        492MB ± 0%        490MB ± 0%  -0.41%  (p=0.000 n=20+19)
SSA            1.40GB ± 0%       1.39GB ± 0%  -0.48%  (p=0.000 n=20+20)
Flate          24.9MB ± 0%       24.9MB ± 0%  -0.24%  (p=0.000 n=20+20)
GoParser       30.9MB ± 0%       30.8MB ± 0%  -0.39%  (p=0.000 n=20+20)
Reflect        77.1MB ± 0%       76.8MB ± 0%  -0.32%  (p=0.000 n=17+20)
Tar            39.1MB ± 0%       39.0MB ± 0%  -0.23%  (p=0.000 n=20+20)
XML            44.7MB ± 0%       44.6MB ± 0%  -0.30%  (p=0.000 n=20+18)

name      old allocs/op     new allocs/op     delta
Template         385k ± 0%         382k ± 0%  -0.99%  (p=0.000 n=20+19)
Unicode          336k ± 0%         336k ± 0%  -0.08%  (p=0.000 n=19+17)
GoTypes         1.20M ± 0%        1.18M ± 0%  -1.11%  (p=0.000 n=20+18)
Compiler        4.66M ± 0%        4.59M ± 0%  -1.42%  (p=0.000 n=19+20)
SSA             11.6M ± 0%        11.5M ± 0%  -1.49%  (p=0.000 n=20+20)
Flate            237k ± 0%         235k ± 0%  -1.00%  (p=0.000 n=20+19)
GoParser         319k ± 0%         315k ± 0%  -1.12%  (p=0.000 n=20+20)
Reflect          960k ± 0%         952k ± 0%  -0.92%  (p=0.000 n=18+20)
Tar              394k ± 0%         390k ± 0%  -0.87%  (p=0.000 n=20+20)
XML              418k ± 0%         413k ± 0%  -1.18%  (p=0.000 n=20+20)

Change-Id: I01b9f45b161379967d7a52e23f39ac30dd90edb0
Reviewed-on: https://go-review.googlesource.com/104415Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>