openpgp: Implement compressed data packets & add support for compressing data during symmetric encryption.

This patch implements the facilities needed to compress data before
encryption as allowed (and recommended) by RFC 4880. The new
functionality is then used to add support for compressing data during
symmetric encryption (openpgp.SymmetricallyEncrypt()).

For now, compression defaults to off. Also, only the ZIP and ZLIB
compression schemes are supported by this patch.

Resulting output tested/verified using GPG.

https://gist.github.com/marete/6189760 is a small program that can be
used to test that the output of various compression/encryption settings
can be read by GPG or other RFC 4880 programs.

Upon review, I will follow this patch with 2 others: a) Add support for 	compression during public key encryption (openpgp.Encrypt()) b) Enable
compression by default (subject to the restrictions of the "Compression 	Preferences" section in RFC 4880).

R=golang-dev, agl
CC=golang-dev
https://golang.org/cl/12685044

openpgp/write.go

@@ -118,11 +118,24 @@ func SymmetricallyEncrypt(ciphertext io.Writer, passphrase []byte, hints *FileHi
 	if err != nil {
 		return
 	}
+
+	literaldata := w
+	if algo := config.Compression(); algo != packet.CompressionNone {
+		var compConfig *packet.CompressionConfig
+		if config != nil {
+			compConfig = config.CompressionConfig
+		}
+		literaldata, err = packet.SerializeCompressed(w, algo, compConfig)
+		if err != nil {
+			return
+		}
+	}
+
 	var epochSeconds uint32
 	if !hints.ModTime.IsZero() {
 		epochSeconds = uint32(hints.ModTime.Unix())
 	}
-	return packet.SerializeLiteral(w, hints.IsBinary, hints.FileName, epochSeconds)
+	return packet.SerializeLiteral(literaldata, hints.IsBinary, hints.FileName, epochSeconds)
 }
 
 // intersectPreferences mutates and returns a prefix of a that contains only