libctf: dedup: add strtab deduplicator

libctf: dedup: add strtab deduplicator

This is a pretty simple two-phase process (count duplicates that are
actually going to end up in the strtab and aren't e.g. strings without refs,
strings with external refs etc, and move them into the parent) with one
wrinkle: we sorta-abuse the csa_external_offset field in the deduplicated
child atom (normally used to indicate that this string is located in the ELF
strtab) to indicate that this atom is in the *parent*.  If you think of
"external" as meaning simply "is in some other strtab, we don't care which
one", this still makes enough sense to not need to change the name, I hope.

This is still not called from anywhere, so strings are (still!) not
deduplicated, and none of the dedup machinery added in earlier commits does
anything yet.

libctf/
	* ctf-dedup.c (ctf_dedup_emit_struct_members): Note that strtab
	dedup happens (well) after struct member emission.
	(ctf_dedup_strings): New.
	* ctf-impl.h (ctf_dedup_strings): Declare.