No tests yet, no documentation update, no :lua* stuff, no vim module.

converter.c should also work with typval_T, not Object.

Known problem: luaeval("1", {}) results in

    PANIC: unprotected error in call to Lua API (attempt to index a nil value)

Ref #3823

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.

atomic multi request for async remote plugins

remove unused response_id parameter of handle_nvim_... helpers

Remove max_fname_len check, which caused false successful lookups,
and was an optimization for a very rare case.