Rollup merge of #42717 - ollie27:into_to_from2, r=sfackler
Rollup merge of #42717 - ollie27:into_to_from2, r=sfackler

Convert `Into<Box<[T]>> for Vec<T>` into `From<Vec<T>> for Box<[T]>`

As the `collections` crate has been merged into `alloc` in #42648 this impl is now possible. This is the final part of #42129 missing from #42227.

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