Make `xilem::Pod` more flexible #2596

Job	Run time
cargo check (msrv) (wasm32)	16s
cargo check (aarch64-android)	19s
cargo check (msrv) (windows-latest)	1m 20s
cargo clippy (windows-latest)	4m 14s
cargo clippy (wasm32)	1m 56s
cargo clippy (macos-latest)	2m 13s
cargo doc	42s
cargo check (msrv) (macos-latest)	33s
cargo clippy (ubuntu-latest)	3m 18s
cargo check (msrv) (ubuntu-latest)	36s
formatting	37s
Prime LFS Cache	4s
typos	4s
cargo test (wasm32)	17s
cargo test (windows-latest)	3m 20s
cargo test (macos-latest)	1m 51s
cargo test (ubuntu-latest)	3m 39s
	25m 19s

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