Despite the removal of the separate movable ref list, the ref system as
a whole is more than complex enough to be worth generalizing now that
we are adding different kinds of ref.
Refs now are lists of uint32_t * which can be updated through the
pointer for all entries in the list and moved to new sites for all
pointers in a given range: they are no longer references to string
offsets in particular and can be references to other uint32_t-sized
things instead (note that ctf_id_t is a typedef to a uint32_t).
ctf-string.c has been adjusted accordingly (the adjustments are tiny,
more or less just turning a bunch of references to atom into
&atom->csa_refs).
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