cmd/compile: reduce allocs to c85b77c (pre-fmt.go change) levels

Linker and reflect info generation (reflect.go) relies on formatting
of types (tconv). The fmt.Format based approach introduces extra
allocations, which matter in those cases. Resurrected sconv and tconv
code from commit c85b77c (fmt.go only); and adjusted it slightly.
The formatter-based approach is still used throughout the rest of the
compiler, but reflect.go now uses the tconv method that simply returns
the desired string.

(The timing data below may not be accurate; I've included it only for
comparison with the numbers in issue #16897).

name       old time/op     new time/op     delta
Template       297ms ± 2%      288ms ± 3%  -3.12%        (p=0.000 n=27+29)
Unicode        155ms ± 5%      150ms ± 5%  -3.26%        (p=0.000 n=30+30)
GoTypes        1.00s ± 3%      0.95s ± 3%  -4.51%        (p=0.000 n=28+29)

name       old alloc/op    new alloc/op    delta
Template      46.8MB ± 0%     46.5MB ± 0%  -0.65%        (p=0.000 n=28+30)
Unicode       37.9MB ± 0%     37.8MB ± 0%  -0.24%        (p=0.000 n=29+30)
GoTypes        144MB ± 0%      143MB ± 0%  -0.68%        (p=0.000 n=30+30)

name       old allocs/op   new allocs/op   delta
Template        469k ± 0%       446k ± 0%  -5.01%        (p=0.000 n=29+30)
Unicode         375k ± 0%       369k ± 0%  -1.62%        (p=0.000 n=30+28)
GoTypes        1.47M ± 0%      1.37M ± 0%  -6.29%        (p=0.000 n=30+30)

The code for sconv/tconv in fmt.go now closely match the code from c85b77c
again; except that the functions are now methods. Removing the use of
the bytes.Buffer in tconv and special-caseing interface{} has helped a
small amount as well:

name       old time/op     new time/op     delta
Template       299ms ± 3%      288ms ± 3%  -3.83%        (p=0.000 n=29+29)
Unicode        156ms ± 5%      150ms ± 5%  -3.56%        (p=0.000 n=30+30)
GoTypes        960ms ± 2%      954ms ± 3%  -0.58%        (p=0.037 n=26+29)

name       old alloc/op    new alloc/op    delta
Template      46.6MB ± 0%     46.5MB ± 0%  -0.22%        (p=0.000 n=30+30)
Unicode       37.8MB ± 0%     37.8MB ± 0%    ~           (p=0.075 n=30+30)
GoTypes        143MB ± 0%      143MB ± 0%  -0.31%        (p=0.000 n=30+30)

name       old allocs/op   new allocs/op   delta
Template        447k ± 0%       446k ± 0%  -0.28%        (p=0.000 n=30+30)
Unicode         369k ± 0%       369k ± 0%  -0.03%        (p=0.032 n=30+28)
GoTypes        1.38M ± 0%      1.37M ± 0%  -0.35%        (p=0.000 n=29+30)

Comparison between c85b77c and now (see issue #16897):

name       old time/op     new time/op     delta
Template       307ms ± 4%      288ms ± 3%  -6.24%  (p=0.000 n=29+29)
Unicode        164ms ± 4%      150ms ± 5%  -8.20%  (p=0.000 n=30+30)
GoTypes        1.01s ± 3%      0.95s ± 3%  -5.72%  (p=0.000 n=30+29)

name       old alloc/op    new alloc/op    delta
Template      46.8MB ± 0%     46.5MB ± 0%  -0.66%  (p=0.000 n=29+30)
Unicode       37.8MB ± 0%     37.8MB ± 0%  -0.13%  (p=0.000 n=30+30)
GoTypes        143MB ± 0%      143MB ± 0%  -0.11%  (p=0.000 n=30+30)

name       old allocs/op   new allocs/op   delta
Template        444k ± 0%       446k ± 0%  +0.48%  (p=0.000 n=30+30)
Unicode         369k ± 0%       369k ± 0%  +0.09%  (p=0.000 n=30+28)
GoTypes        1.35M ± 0%      1.37M ± 0%  +1.47%  (p=0.000 n=30+30)

There's still a small increase (< 1.5%) for GoTypes but pending a complete
rewrite of fmt.go, this seems ok again.

Fixes #16897.

Change-Id: I7e0e56cd1b9f981252eded917f5752259d402354
Reviewed-on: https://go-review.googlesource.com/29087
Run-TryBot: Robert Griesemer <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Matthew Dempsky <[email protected]>

Author: griesemer