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 <mvdan@mvdan.cc>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/xml/xml.go

@@ -806,18 +806,7 @@ func (d *Decoder) rawToken() (Token, error) {
 		}
 		d.ungetc(b)
 
-		n := len(attr)
+		a := Attr{}
-		if n >= cap(attr) {
-			nCap := 2 * cap(attr)
-			if nCap == 0 {
-				nCap = 4
-			}
-			nattr := make([]Attr, n, nCap)
-			copy(nattr, attr)
-			attr = nattr
-		}
-		attr = attr[0 : n+1]
-		a := &attr[n]
 		if a.Name, ok = d.nsname(); !ok {
 			if d.err == nil {
 				d.err = d.syntaxError("expected attribute name in element")
@@ -843,6 +832,7 @@ func (d *Decoder) rawToken() (Token, error) {
 			}
 			a.Value = string(data)
 		}
+		attr = append(attr, a)
 	}
 	if empty {
 		d.needClose = true