rv is int64_t but its address is being passed into win_get_tabwin as if
it were an int.  This breaks on big-endian systems, since win_get_tabwin
will store the data to the "wrong" half of the int64_t, thus returning
invalid data out of nvim_win_get_number.

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.

API level is disconnected from NVIM version. The API metadata holds the
current API level, and the lowest backwards-compatible level supported
by this instance.

Release 0.1.6 is the first release that reports the Nvim version and API
level.

    metadata['version'] = {
      major: 0,
      minor: 1,
      patch: 6,
      api_level: 1,
      api_compatible: 0,
      api_prerelease: false,
    }

The API level may remain unchanged across Nvim releases if the API has
not changed.

When changing the API,
    - set NVIM_API_PRERELEASE to true
    - increment NVIM_API_LEVEL (at most once per Nvim version)
    - adjust NVIM_API_LEVEL_COMPAT if backwards-compatibility was broken

api_level_0.mpack was generated from Nvim 0.1.5 with:
    nvim --api-info

The API level is disconnected from the NVIM version. The API metadata
holds the current API level, and the lowest backwards-compatible level
supported by this instance.

Release 0.1.6 will be the first release reporting the Nvim version and
API level.

    metadata['version'] = {
      major: 0,
      minor: 1,
      patch: 6,
      prerelease: true,
      api_level: 1,
      api_compatible: 0,
    }

The API level may remain unchanged across Neovim releases if the API has
not changed.

When changing the API the CMake variable NVIM_API_PRERELEASE is set to
true, and  NVIM_API_CURRENT/NVIM_API_COMPATIBILITY are incremented
accordingly.

The functional tests check the API table against fixtures of past
versions of Neovim. It compares all the functions in the old table with
the new one, it does ignore some metadata attributes that do not alter
the function signature or were removed since 0.1.5.  Currently the only
fixture is 0.mpack, generated from Neovim 0.1.5 with nvim --api-info.

TODO: Also spec behavior of Press-Enter prompt for these API functions.

Clarify behavior of v:errmsg and the Error object.

atomic multi request for async remote plugins

remove unused response_id parameter of handle_nvim_... helpers