bytes: rely on runtime.growslice for growing

Rather than naively making a slice of capacity 2*c+n,
rely on the append(..., make(...)) pattern to allocate a
slice that aligns up to the closest size class.

Performance:
	name                          old time/op    new time/op    delta
	BufferWriteBlock/N4096       3.03µs ± 6%    2.04µs ± 6%  -32.60%  (p=0.000 n=10+10)
	BufferWriteBlock/N65536      47.8µs ± 6%    28.1µs ± 2%  -41.32%  (p=0.000 n=9+8)
	BufferWriteBlock/N1048576     844µs ± 7%     510µs ± 5%  -39.59%  (p=0.000 n=8+9)

	name                          old alloc/op   new alloc/op   delta
	BufferWriteBlock/N4096       12.3kB ± 0%     7.2kB ± 0%  -41.67%  (p=0.000 n=10+10)
	BufferWriteBlock/N65536       258kB ± 0%     130kB ± 0%  -49.60%  (p=0.000 n=10+10)
	BufferWriteBlock/N1048576    4.19MB ± 0%    2.10MB ± 0%  -49.98%  (p=0.000 n=10+8)

	name                          old allocs/op  new allocs/op  delta
	BufferWriteBlock/N4096         3.00 ± 0%      3.00 ± 0%     ~     (all equal)
	BufferWriteBlock/N65536        7.00 ± 0%      7.00 ± 0%     ~     (all equal)
	BufferWriteBlock/N1048576      11.0 ± 0%      11.0 ± 0%     ~     (all equal)

The performance is faster since the growth rate is capped at 2x,
while previously it could grow by amounts potentially much greater than 2x,
leading to significant amounts of memory waste and extra copying.

Credit goes to Martin Möhrmann for suggesting the
append(b, make([]T, n)...) pattern.

Fixes #42984
Updates #51462

Change-Id: I7b23f75dddbf53f8b8b93485bb1a1fff9649b96b
Reviewed-on: https://go-review.googlesource.com/c/go/+/349994
Trust: Joseph Tsai <joetsai@digital-static.net>
Trust: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/bytes/buffer_test.go

@@ -672,3 +672,18 @@ func BenchmarkBufferFullSmallReads(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkBufferWriteBlock(b *testing.B) {
+	block := make([]byte, 1024)
+	for _, n := range []int{1 << 12, 1 << 16, 1 << 20} {
+		b.Run(fmt.Sprintf("N%d", n), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				var bb Buffer
+				for bb.Len() < n {
+					bb.Write(block)
+				}
+			}
+		})
+	}
+}