Enable -Wconversion: fold.c.

Refactor summary:
- foldinfo_T.fi_lnum: int --> linenr_T
                      Reorder field for optimal packing.
- foldAddMarker(..., markerlen): int --> size_t
    * foldstartmarkerlen: int --> size_t
        - foldDelMarker(..., markerlen): int --> size_t
            * foldendmarkerlen: int --> size_t

Helped-by: oni-link <knil.ino@gmail.com>

src/nvim/fold.h

@@ -1,14 +1,16 @@
 #ifndef NVIM_FOLD_H
 #define NVIM_FOLD_H
 
+#include "nvim/pos.h"
+
 /*
  * Info used to pass info about a fold from the fold-detection code to the
  * code that displays the foldcolumn.
  */
 typedef struct foldinfo {
+  linenr_T fi_lnum;             /* line number where fold starts */
   int fi_level;                 /* level of the fold; when this is zero the
                                    other fields are invalid */
-  int fi_lnum;                  /* line number where fold starts */
   int fi_low_level;             /* lowest fold level that starts in the same
                                    line */
 } foldinfo_T;