encoding/asn1: reduce allocations in Marshal

Current code uses trees of bytes.Buffer as data representation.
Each bytes.Buffer takes 4k bytes at least, so it's waste of memory.
The change introduces trees of lazy-encoder as
alternative one which reduce allocations.

name       old time/op    new time/op    delta
Marshal-4    64.7µs ± 2%    42.0µs ± 1%  -35.07%   (p=0.000 n=9+10)

name       old alloc/op   new alloc/op   delta
Marshal-4    35.1kB ± 0%     7.6kB ± 0%  -78.27%  (p=0.000 n=10+10)

name       old allocs/op  new allocs/op  delta
Marshal-4       503 ± 0%       293 ± 0%  -41.75%  (p=0.000 n=10+10)

Change-Id: I32b96c20b8df00414b282d69743d71a598a11336
Reviewed-on: https://go-review.googlesource.com/27030Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/encoding/asn1/asn1_test.go

@@ -132,9 +132,9 @@ func TestParseBigInt(t *testing.T) {
 			if ret.String() != test.base10 {
 				t.Errorf("#%d: bad result from %x, got %s want %s", i, test.in, ret.String(), test.base10)
 			}
-			fw := newForkableWriter()
-			marshalBigInt(fw, ret)
-			result := fw.Bytes()
+			e := makeBigInt(ret)
+			result := make([]byte, e.Len())
+			e.Encode(result)
 			if !bytes.Equal(result, test.in) {
 				t.Errorf("#%d: got %x from marshaling %s, want %x", i, result, ret, test.in)
 			}

src/encoding/asn1/marshal_test.go

@@ -173,3 +173,13 @@ func TestInvalidUTF8(t *testing.T) {
 		t.Errorf("invalid UTF8 string was accepted")
 	}
 }
+
+func BenchmarkMarshal(b *testing.B) {
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		for _, test := range marshalTests {
+			Marshal(test.in)
+		}
+	}
+}