crypto/tls: implement TLS 1.3 client handshake (base)

Implement a basic TLS 1.3 client handshake, only enabled if explicitly
requested with MaxVersion.

This CL intentionally leaves for future CLs:
  - PSK modes and resumption
  - client authentication
  - post-handshake messages
  - downgrade protection
  - KeyLogWriter support

Updates #9671

Change-Id: Ieb6130fb6f25aea4f0d39e3a2448dfc942e1de7a
Reviewed-on: https://go-review.googlesource.com/c/146559
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

cipher_suites.go

@@ -410,6 +410,20 @@ func mutualCipherSuite(have []uint16, want uint16) *cipherSuite {
 	return nil
 }
 
+func mutualCipherSuiteTLS13(have []uint16, want uint16) *cipherSuiteTLS13 {
+	for _, id := range have {
+		if id == want {
+			for _, suite := range cipherSuitesTLS13 {
+				if suite.id == want {
+					return suite
+				}
+			}
+			return nil
+		}
+	}
+	return nil
+}
+
 // A list of cipher suite IDs that are, or have been, implemented by this
 // package.
 //