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/os/time.h

@@ -5,6 +5,8 @@
 #include <stdbool.h>
 #include <time.h>
 
+typedef time_t Timestamp;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "os/time.h.generated.h"
 #endif