Generalise strbuf_catf() into put_fmt().

marshal.h now provides a macro put_fmt() which allows you to write
arbitrary printf-formatted data to an arbitrary BinarySink.

We already had this facility for strbufs in particular, in the form of
strbuf_catf(). That was able to take advantage of knowing the inner
structure of a strbuf to minimise memory allocation (it would snprintf
directly into the strbuf's existing buffer if possible). For a general
black-box BinarySink we can't do that, so instead we dupvprintf into a
temporary buffer.

For consistency, I've removed strbuf_catf, and converted all uses of
it into the new put_fmt - and I've also added an extra vtable method
in the BinarySink API, so that put_fmt can still use strbuf_catf's
more efficient memory management when talking to a strbuf, and fall
back to the simpler strategy when that's not available.

misc.h

@@ -56,8 +56,6 @@ void *strbuf_append(strbuf *buf, size_t len);
 void strbuf_shrink_to(strbuf *buf, size_t new_len);
 void strbuf_shrink_by(strbuf *buf, size_t amount_to_remove);
 char *strbuf_to_str(strbuf *buf); /* does free buf, but you must free result */
-void strbuf_catf(strbuf *buf, const char *fmt, ...) PRINTF_LIKE(2, 3);
-void strbuf_catfv(strbuf *buf, const char *fmt, va_list ap);
 static inline void strbuf_clear(strbuf *buf) { strbuf_shrink_to(buf, 0); }
 bool strbuf_chomp(strbuf *buf, char char_to_remove);