libctf: create: vlen growth and prefix addition (NEEDS REVIEW)
This commit modifies ctf_grow_vlen to account for the recent changes to
ctf_dtdef_t, and adds a new ctf_add_prefix function to add a prefix to an
existing type, moving the dtd_data and dtd_vlen up accordinly.
It deserves close review, since this is probably the single greatest bug
cluster in libctf: the number of times I added to a variable of type
ctf_type_t and assumed it would move it in bytes rather than ctf_type_t
units is hard to believe.