viminfo: First version of ShaDa file dumping

What works:

1. ShaDa file dumping: header, registers, jump list, history, search patterns,
   substitute strings, variables.
2. ShaDa file reading: registers, global marks, variables.

Most was not tested.

TODO:

1. Merging.
2. Reading history, local marks, jump and buffer lists.
3. Documentation update.
4. Converting some data from &encoding.
5. Safer variant of dumping viminfo (dump to temporary file then rename).
6. Removing old viminfo code (currently masked with `#if 0` in a ShaDa file for
   reference).

src/nvim/mark_defs.h

@@ -2,25 +2,41 @@
 #define NVIM_MARK_DEFS_H
 
 #include "nvim/pos.h"
+#include "nvim/os/time.h"
+#include "nvim/api/private/defs.h"
 
 /*
  * marks: positions in a file
  * (a normal mark is a lnum/col pair, the same as a file position)
  */
 
-#define NMARKS          ('z' - 'a' + 1) /* max. # of named marks */
-#define JUMPLISTSIZE    100             /* max. # of marks in jump list */
-#define TAGSTACKSIZE    20              /* max. # of tags in tag stack */
+/// Number of possible numbered global marks
+#define EXTRA_MARKS     ('9' - '0' + 1)
 
+/// Maximum possible number of letter marks
+#define NMARKS          ('z' - 'a' + 1)
+
+/// Total possible number of global marks
+#define NGLOBALMARKS    (NMARKS + EXTRA_MARKS)
+
+/// Maximum number of marks in jump list
+#define JUMPLISTSIZE    100
+
+/// Maximum number of tags in tag stack
+#define TAGSTACKSIZE    20
+
+/// Structure defining single local mark
 typedef struct filemark {
-  pos_T mark;                   /* cursor position */
-  int fnum;                     /* file number */
+  pos_T mark;           ///< Cursor position.
+  int fnum;             ///< File number.
+  Timestamp timestamp;  ///< Time when this mark was last set.
+  Dictionary *additional_data;  ///< Additional data from ShaDa file.
 } fmark_T;
 
-/* Xtended file mark: also has a file name */
+/// Structure defining extended mark (mark with file name attached)
 typedef struct xfilemark {
-  fmark_T fmark;
-  char_u      *fname;           /* file name, used when fnum == 0 */
+  fmark_T fmark;       ///< Actual mark.
+  char_u      *fname;  ///< File name, used when fnum == 0.
 } xfmark_T;
 
 #endif // NVIM_MARK_DEFS_H