don't force allocate for most Unsize casts
don't force allocate for most Unsize casts

only Arc<T> -> Arc<Trait> unsize casts are left
rust-lang/miri,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,rust-lang/miri,graydon/rust,tsion/miri,aidancully/rust,GBGamer/rust,rust-lang/miri,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,rust-lang/miri,aidancully/rust,GBGamer/rust,graydon/rust