c_vec introduction form must be unsafe

[jwise]

5244512 appears to have introduced a regression in which the introduction forms for c_vec are no longer marked as unsafe functions. They should be marked as unsafe. (See 5244512#L0L87 .)

(I would come up with a pull request myself, but I haven't a checkout of HEAD -- only 0.6.)

[jwise]

Closed by #6339; thanks!