Add BinarySink wrappers on existing forms of output.

There's now a stdio_sink, whose write function calls fwrite on the
given FILE *; a bufchain_sink, whose write function appends to the
given bufchain; and on Windows there's a handle_sink whose write
function writes to the given 'struct handle'. (That is, not the raw
Windows HANDLE, but our event-loop-friendly wrapper on it.)

Not yet used for anything, but they're about to be.

defs.h

@@ -40,6 +40,9 @@ typedef struct RSAKey RSAKey;
 
 typedef struct BinarySink BinarySink;
 typedef struct BinarySource BinarySource;
+typedef struct stdio_sink stdio_sink;
+typedef struct bufchain_sink bufchain_sink;
+typedef struct handle_sink handle_sink;
 
 typedef struct IdempotentCallback IdempotentCallback;