Make `xilem::Pod` more flexible #2591

Job	Run time
cargo check (msrv) (windows-latest)	1m 38s
cargo check (msrv) (wasm32)	20s
cargo check (msrv) (macos-latest)	39s
cargo clippy (windows-latest)	31s
cargo check (aarch64-android)	24s
cargo check (msrv) (ubuntu-latest)	40s
cargo clippy (macos-latest)	29s
cargo clippy (ubuntu-latest)	30s
cargo clippy (wasm32)	1m 55s
cargo doc	1m 54s
formatting	35s
cargo test (wasm32)	20s
Prime LFS Cache	4s
typos	5s
cargo test (windows-latest)	4m 19s
cargo test (macos-latest)	1m 45s
cargo test (ubuntu-latest)	29s
	16m 37s

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