encoder/Interface: move instance methods to abstract class

Rename struct Encoder to PreparedEncoder, and add a new (abstract)
class Encoder which represents one encoder instance.

src/encoder/ToOutputStream.hxx

@@ -22,8 +22,8 @@
 
 #include "check.h"
 
-struct Encoder;
 class OutputStream;
+class Encoder;
 
 void
 EncoderToOutputStream(OutputStream &os, Encoder &encoder);