go.crypto/ssh: improve marshal performance

Atom N450, 6g

benchmark                         old ns/op    new ns/op    delta
BenchmarkMarshalKexInitMsg            96446        66675  -30.87%
BenchmarkUnmarshalKexInitMsg         155341       142715   -8.13%
BenchmarkMarshalKexDHInitMsg           9499         8340  -12.20%
BenchmarkUnmarshalKexDHInitMsg         4973         5145   +3.46%

Intel E3-1270, 6g

benchmark                         old ns/op    new ns/op    delta
BenchmarkMarshalKexInitMsg            23218        16903  -27.20%
BenchmarkUnmarshalKexInitMsg          31384        31640   +0.82%
BenchmarkMarshalKexDHInitMsg           1943         1661  -14.51%
BenchmarkUnmarshalKexDHInitMsg          915          941   +2.84%

R=agl, minux.ma, remyoudompheng
CC=golang-dev
https://golang.org/cl/5728053

ssh/common.go

@@ -249,3 +249,15 @@ func safeString(s string) string {
 	}
 	return string(out)
 }
+
+func appendU16(buf []byte, n uint16) []byte {
+	return append(buf, byte(n>>8), byte(n))
+}
+
+func appendU32(buf []byte, n uint32) []byte {
+	return append(buf, byte(n>>24), byte(n>>16), byte(n>>8), byte(n))
+}
+
+func appendInt(buf []byte, n int) []byte {
+	return appendU32(buf, uint32(n))
+}