Redo toc.c to add the new function mkd_toc(), which writes the

generate toc to a string.   Documented in `markdown.3`
and `mkd-functions.3`, requires a new file (`Csio.c`) that has
functions for writing to a Cstring, requires that some of the
internal functions in `generate.c` be exposed (and renaed to
`__mkd_`*function* to reduce namespace collisions.

The new functions in Csio (particularly `Csprintf()` should make
it easier to convert many of the other "we print to a `FILE*` and
you need to lump it" functions into a pair of "print to a string"
and "print to a file" functions.

cstring.h

@@ -50,6 +50,7 @@
  */
 #define T(x)		(x).text
 #define S(x)		(x).size
+#define ALL(x)		(x).alloc
 
 /* abstract anchor type that defines a list base
  * with a function that attaches an element to
@@ -65,4 +66,7 @@
 
 typedef STRING(char) Cstring;
 
+extern int Csputc(int, Cstring *);
+extern int Csprintf(Cstring *, char *, ...);
+
 #endif/*_CSTRING_D*/