New encrypted socket API

[thekid]

This pull request deprecates the CryptoSocket class and its two implementations, SSLSocket and TLSSocket, in favor of a new class peer.EncryptedSocket, combining all of the functionality into one class.

Old code

use peer\{SSLSocket, TLSSocket};

// TLSSocket's constructor won't let us set the method directly,
// modify the base class' public member instead (!)
$newTLSSocket= function($host, $port, $method) {
  $s= new TLSSocket($host, $port);
  $s->cryptoImpl= $method;
  return $s;
};

$socket= match ($method) {
  'ssl'    => new SSLSocket($host, $port),
  'sslv2'  => new SSLSocket($host, $port, null, 2),
  'sslv3'  => new SSLSocket($host, $port, null, 3),
  'tls'    => new TLSSocket($host, $port),
  'tlsv10' => $newTLSSocket($host, $port, STREAM_CRYPTO_METHOD_TLSv1_0_CLIENT),
  'tlsv11' => // ...shortened for brevity
  default  => throw new IllegalArgumentException('Unknown crypto method '.$method),
};

New code

use peer\EncryptedSocket;

$socket= new EncryptedSocket($host, $port, null, $method);

[thekid]

Alternatively, we could add a method encrypt() to the base class