xilem: Add a way to allow `impl WidgetViewSequence` as `<seq>` in `flex(<seq>)` #1354

Job	Run time
cargo check (msrv) (wasm32)	22s
cargo clippy (windows-latest)	2m 17s
cargo check (aarch64-android)	20s
cargo check (msrv) (windows-latest)	1m 20s
cargo clippy (macos-latest)	1m 5s
cargo check (msrv) (macos-latest)	32s
cargo clippy (ubuntu-latest)	1m 22s
cargo clippy (wasm32)	1m 1s
cargo check (msrv) (ubuntu-latest)	36s
cargo doc	1m 19s
formatting	30s
Prime LFS Cache	4s
typos	4s
cargo test (wasm32)	18s
cargo test (windows-latest)	3m 15s
cargo test (macos-latest)	1m 11s
cargo test (ubuntu-latest)	3m 7s
	18m 43s

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