Rollup merge of #130624 - theemathas:vec_as_non_null, r=Noratrieb

Add `Vec::as_non_null`

Implements the ACP: rust-lang/libs-team#440

The documentation is mostly copied from the existing `Vec::as_mut_ptr` method.

I am adding this method to the already-existing `box_vec_non_null` feature tracked at rust-lang/rust#130364.