vec: More specialization for Extend<&T> for vec
vec: More specialization for Extend<&T> for vec

Specialize to use copy_from_slice when extending a Vec with &[T] where
T: Copy.

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