object_to_vim: Fix buffer/window/tabpage conversion on BE systems

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.

src/nvim/api/private/defs.h

@@ -91,9 +91,6 @@ typedef enum {
 struct object {
   ObjectType type;
   union {
-    Buffer buffer;
-    Window window;
-    Tabpage tabpage;
     Boolean boolean;
     Integer integer;
     Float floating;