Escape analysis has a hard time with tree-like
structures (see #13493 and #14858).
This is unlikely to change.
As a result, when invoking a function that accepts
a **Node parameter, we usually allocate a *Node
on the heap. This happens a whole lot.

This CL changes functions from taking a **Node
to acting more like append: It both modifies
the input and returns a replacement for it.

Because of the cascading nature of escape analysis,
in order to get the benefits, I had to modify
almost all such functions. The remaining functions
are in racewalk and the backend. I would be happy
to update them as well in a separate CL.

This CL was created by manually updating the
function signatures and the directly impacted
bits of code. The callsites were then automatically
updated using a bespoke script:
https://gist.github.com/josharian/046b1be7aceae244de39

For ease of reviewing and future understanding,
this CL is also broken down into four CLs,
mailed separately, which show the manual
and the automated changes separately.
They are CLs 20990, 20991, 20992, and 20993.

Passes toolstash -cmp.

name       old time/op     new time/op     delta
Template       335ms ± 5%      324ms ± 5%   -3.35%        (p=0.000 n=23+24)
Unicode        176ms ± 9%      165ms ± 6%   -6.12%        (p=0.000 n=23+24)
GoTypes        1.10s ± 4%      1.07s ± 2%   -2.77%        (p=0.000 n=24+24)
Compiler       5.31s ± 3%      5.15s ± 3%   -2.95%        (p=0.000 n=24+24)
MakeBash       41.6s ± 1%      41.7s ± 2%     ~           (p=0.586 n=23+23)

name       old alloc/op    new alloc/op    delta
Template      63.3MB ± 0%     62.4MB ± 0%   -1.36%        (p=0.000 n=25+23)
Unicode       42.4MB ± 0%     41.6MB ± 0%   -1.99%        (p=0.000 n=24+25)
GoTypes        220MB ± 0%      217MB ± 0%   -1.11%        (p=0.000 n=25+25)
Compiler       994MB ± 0%      973MB ± 0%   -2.08%        (p=0.000 n=24+25)

name       old allocs/op   new allocs/op   delta
Template        681k ± 0%       574k ± 0%  -15.71%        (p=0.000 n=24+25)
Unicode         518k ± 0%       413k ± 0%  -20.34%        (p=0.000 n=25+24)
GoTypes        2.08M ± 0%      1.78M ± 0%  -14.62%        (p=0.000 n=25+25)
Compiler       9.26M ± 0%      7.64M ± 0%  -17.48%        (p=0.000 n=25+25)

name       old text-bytes  new text-bytes  delta
HelloSize       578k ± 0%       578k ± 0%     ~     (all samples are equal)
CmdGoSize      6.46M ± 0%      6.46M ± 0%     ~     (all samples are equal)

name       old data-bytes  new data-bytes  delta
HelloSize       128k ± 0%       128k ± 0%     ~     (all samples are equal)
CmdGoSize       281k ± 0%       281k ± 0%     ~     (all samples are equal)

name       old exe-bytes   new exe-bytes   delta
HelloSize       921k ± 0%       921k ± 0%     ~     (all samples are equal)
CmdGoSize      9.86M ± 0%      9.86M ± 0%     ~     (all samples are equal)

Change-Id: I277d95bd56d51c166ef7f560647aeaa092f3f475
Reviewed-on: https://go-review.googlesource.com/20959Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Ian Lance Taylor <iant@golang.org>