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/mbyte.c

@@ -66,7 +66,7 @@
  * (4) The encoding of the file is specified with 'fileencoding'.  Conversion
  *     is to be done when it's different from 'encoding'.
  *
- * The viminfo file is a special case: Only text is converted, not file names.
+ * The ShaDa file is a special case: Only text is converted, not file names.
  * Vim scripts may contain an ":encoding" command.  This has an effect for
  * some commands, like ":menutrans"
  */