Ruby

make overloaded_cme_table truly weak key map

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.