This reduces the amount of memory copied when creating/deleting a
variable on an object. We now use a table of pointers instead of table
of variable_t.

This also simplifies callback handling a bit as the variable_t cannot
move anymore while we wait. Earlier another thread could add or remove
another variable on the same object, thus changing the variable table.

With this change, we could also store non-serialized data (i.e.
non-movable with memcpy/memmove) directly inside variable_t.