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/helpers.h

@@ -37,15 +37,15 @@
 
 #define BUFFER_OBJ(s) ((Object) { \
     .type = kObjectTypeBuffer, \
-    .data.buffer = s })
+    .data.integer = s })
 
 #define WINDOW_OBJ(s) ((Object) { \
     .type = kObjectTypeWindow, \
-    .data.window = s })
+    .data.integer = s })
 
 #define TABPAGE_OBJ(s) ((Object) { \
     .type = kObjectTypeTabpage, \
-    .data.tabpage = s })
+    .data.integer = s })
 
 #define ARRAY_OBJ(a) ((Object) { \
     .type = kObjectTypeArray, \