Also fixes the error: when writing shada files existing history items may be
freed.

Warning: valgrind reports some memory leaks.

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).

Arguments passed to xmemdupz() are sometimes NULL, but xmemdupz() has
FUNC_ATTR_NONNULL_ALL. Check pointers for NULL before calling
xmemdupz().

Resolves #2533.

Closes #2816

Consider: `let vim = rpcstart('nvim', ['--embed'])`

Allows `rpcnotify(vim, ...)` to work like an asynchronous
`rpcrequest(nvim, ...)`.

Helped-by: Michael Reed <m.reed@mykolab.com>
Helped-by: Justin M. Keyes <>

We already use wrappers for allocation, the new `xfree` function is the
equivalent for deallocation and provides a way to fully replace the malloc
implementation used by Neovim.

Fixes: #1976