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.h

@@ -4,6 +4,22 @@
 #include "nvim/buffer_defs.h"
 #include "nvim/mark_defs.h"
 #include "nvim/pos.h"
+#include "nvim/os/time.h"
+
+/// Free and set fmark using given value
+#define RESET_FMARK(fmarkp_, mark_, fnum_) \
+    do { \
+      fmark_T *const fmarkp__ = fmarkp_; \
+      free_fmark(*fmarkp__); \
+      fmarkp__->mark = mark_; \
+      fmarkp__->fnum = fnum_; \
+      fmarkp__->timestamp = os_time(); \
+      fmarkp__->additional_data = NULL; \
+    } while (0)
+
+/// Clear given fmark
+#define CLEAR_FMARK(fmarkp_) \
+    RESET_FMARK(fmarkp_, ((pos_T) {0, 0, 0}), 0)
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "mark.h.generated.h"