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/ops.h

@@ -4,6 +4,8 @@
 #include <stdbool.h>
 
 #include "nvim/types.h"
+#include "nvim/api/private/defs.h"
+#include "nvim/os/time.h"
 
 typedef int (*Indenter)(void);
 
@@ -47,14 +49,6 @@ typedef int (*Indenter)(void);
 #define OP_FORMAT2      26      /* "gw" format operator, keeps cursor pos */
 #define OP_FUNCTION     27      /* "g@" call 'operatorfunc' */
 
-/// Contents of a yank (read-write) register
-typedef struct yankreg {
-  char_u      **y_array;        ///< pointer to array of line pointers
-  linenr_T y_size;              ///< number of lines in y_array
-  char_u y_type;                ///< MLINE, MCHAR or MBLOCK
-  colnr_T y_width;              ///< only set if y_type == MBLOCK
-} yankreg_T;
-
 /// Flags for get_reg_contents().
 enum GRegFlags {
   kGRegNoExpr  = 1,  ///< Do not allow expression register.
@@ -62,6 +56,16 @@ enum GRegFlags {
   kGRegList    = 4   ///< Return list.
 };
 
+/// Definition of one register
+typedef struct yankreg {
+  char_u **y_array;  ///< Pointer to an array of line pointers.
+  linenr_T y_size;   ///< Number of lines in y_array.
+  char_u y_type;     ///< Register type: MLINE, MCHAR or MBLOCK.
+  colnr_T y_width;   ///< Register width (only valid for y_type == MBLOCK).
+  Timestamp timestamp;  ///< Time when register was last modified.
+  Dictionary *additional_data;  ///< Additional data from ShaDa file.
+} yankreg_T;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ops.h.generated.h"
 #endif