Restore Vec::from_iter() specialization
Restore Vec::from_iter() specialization

Since I said "no intentional functional change" in the previous commit,
I guess it was inevitable there were unintentional changes. Not
functional, but optimization-wise. This restores the extend
specialization's use in Vec::from_iter.

GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust