Remove transmute from `btree::node::Node::as_slices_internal_mut`
Remove transmute from `btree::node::Node::as_slices_internal_mut`

Closes #27620.

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