xilem_core: Add a `WithoutElements` `ViewSequence` #1598

Job	Run time
cargo check (aarch64-android)	20s
cargo check (msrv) (windows-latest)	1m 56s
cargo check (msrv) (wasm32)	17s
cargo clippy (windows-latest)	3m 37s
cargo check (msrv) (macos-latest)	26s
cargo clippy (macos-latest)	1m 13s
cargo check (msrv) (ubuntu-latest)	36s
cargo clippy (wasm32)	1m 21s
cargo doc	1m 45s
cargo clippy (ubuntu-latest)	1m 38s
formatting	52s
cargo test (wasm32)	23s
typos	4s
Prime LFS Cache	4s
cargo test (windows-latest)	3m 14s
cargo test (macos-latest)	1m 30s
cargo test (ubuntu-latest)	3m 41s
	22m 57s

Provide feedback