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.c

@@ -103,3 +103,12 @@ struct tm *os_get_localtime(struct tm *result) FUNC_ATTR_NONNULL_ALL
   time_t rawtime = time(NULL);
   return os_localtime_r(&rawtime, result);
 }
+
+/// Obtains the current UNIX timestamp
+///
+/// @return Seconds since epoch.
+Timestamp os_time(void)
+  FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  return (Timestamp) time(NULL);
+}