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/api/private/helpers.h

@@ -69,6 +69,8 @@
 #define ADD(array, item)                                                      \
   kv_push(Object, array, item)
 
+#define STATIC_CSTR_AS_STRING(s) ((String) {.data = s, .size = sizeof(s) - 1})
+
 // Helpers used by the generated msgpack-rpc api wrappers
 #define api_init_boolean
 #define api_init_integer