`overloaded_cme_table` keeps cme -> monly_cme pairs to manage
corresponding `monly_cme` for `cme`. The lifetime of the `monly_cme`
should be longer than `monly_cme`, but the previous patch losts the
reference to the living `monly_cme`.

Now `overloaded_cme_table` values are always root (keys are only weak
reference), it means `monly_cme` does not freed until corresponding
`cme` is invalidated.

To make managing easy, move `overloaded_cme_table` to `rb_vm_t`.