Attempting to serialize a NULL string through msgpack results in
msgpack_sbuffer_write attempting to memcpy from a NULL pointer, which is
undefined behavior.

Since data.integer is a different (larger) integer type than
data.{buffer,window,tabpage}, we cannot abuse the union by using
data.integer to access the value for all 4 types.  Instead, remove the
{buffer,window,tabpage} members and always use the integer member.

In order to accomodate this, perform distinct validation and coercion
between the Integer type and Buffer/Window/Tabpage types in
object_to_vim, msgpack_rpc helpers, and gendispatch.lua.

remove unused response_id parameter of handle_nvim_... helpers

`lib/queue.h` implements a basic queue. `event/queue.c` implements
a specialized data structure on top of lib/queue.h; it is not a "normal"
queue.

Rename the specialized multi-level queue implemented in event/queue.c to
"multiqueue", to avoid confusion when reading the code.

Before this change one can eventually notice that "macros (uppercase
symbols) are for the normal queue, lowercase operations are for the
multi-level queue", but that is unnecessary friction for new developers
(or existing developers just visiting this part of the codebase).

This applies both to msgpack-rpc and eval.

also allow handle==0 meaning curbuf/curwin/curtab

This makes stderr and exit callbacks work for rpc jobs