encoding/xml: simplify slice-growing logic in rawToken
encoding/xml: simplify slice-growing logic in rawToken

It appears that old code (from 2009) in xml.(*Decoder).rawToken
replicates append's slice-growing functionality by allocating a new,
bigger backing array and then calling copy.

Simplifying the code by replacing it with a single append call does
not seem to hurt performance:

name         old time/op    new time/op    delta
Marshal-4      11.2µs ± 1%    11.3µs ±10%    ~     (p=0.069 n=19+17)
Unmarshal-4    28.6µs ± 1%    28.4µs ± 1%  -0.60%  (p=0.000 n=20+18)

name         old alloc/op   new alloc/op   delta
Marshal-4      5.78kB ± 0%    5.78kB ± 0%    ~     (all equal)
Unmarshal-4    8.61kB ± 0%    8.27kB ± 0%  -3.90%  (p=0.000 n=20+20)

name         old allocs/op  new allocs/op  delta
Marshal-4        23.0 ± 0%      23.0 ± 0%    ~     (all equal)
Unmarshal-4       189 ± 0%       190 ± 0%  +0.53%  (p=0.000 n=20+20)

Change-Id: Ie580d1216a44760e611e63dee2c339af5465aea5
Reviewed-on: https://go-review.googlesource.com/86655
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

josharian/go,CAFxX/go,josharian/go,josharian/go,akutz/go,golang/go,akutz/go,cloudflare/go,CAFxX/go,Cofyc/go,josharian/go,codestation/go,codestation/go,codestation/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,Cofyc/go,Cofyc/go,Cofyc/go,CAFxX/go,cloudflare/go,codestation/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,josharian/go,akutz/go,cloudflare/go,CAFxX/go,golang/go,golang/go,akutz/go,josharian/go,Cofyc/go,akutz/go,golang/go,nawawi/go,akutz/go,Cofyc/go,nawawi/go,josharian/go,codestation/go,golang/go,josharian/go,akutz/go,Cofyc/go,cloudflare/go,nawawi/go,nawawi/go,codestation/go,codestation/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,cloudflare/go,Cofyc/go,golang/go,codestation/go,nawawi/go