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.

utils/marshal.c

@@ -1,4 +1,5 @@
 #include <assert.h>
+#include <stdarg.h>
 #include <stddef.h>
 #include <string.h>
 
@@ -99,6 +100,25 @@ bool BinarySink_put_pstring(BinarySink *bs, const char *str)
     return true;
 }
 
+void BinarySink_put_fmtv(BinarySink *bs, const char *fmt, va_list ap)
+{
+    if (bs->writefmtv) {
+        bs->writefmtv(bs, fmt, ap);
+    } else {
+        char *str = dupvprintf(fmt, ap);
+        bs->write(bs, str, strlen(str));
+        burnstr(str);
+    }
+}
+
+void BinarySink_put_fmt(BinarySink *bs, const char *fmt, ...)
+{
+    va_list ap;
+    va_start(ap, fmt);
+    BinarySink_put_fmtv(bs, fmt, ap);
+    va_end(ap);
+}
+
 /* ---------------------------------------------------------------------- */
 
 static bool BinarySource_data_avail(BinarySource *src, size_t wanted)