Vec IntoIter: Drop using raw slice
Vec IntoIter: Drop using raw slice

Update Vec drop with a comment to explain why we want to use a raw
slice, and extend this pattern to also include the Vec's IntoIter.

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