xilem_web: Optimize element casting, and intern often used strings (e… #1540

Job	Run time
formatting	27s
cargo clippy (wasm32)	1m 23s
cargo clippy (windows-latest)	4m 35s
cargo clippy (macos-latest)	1m 36s
cargo clippy (ubuntu-latest)	2m 8s
cargo check (msrv) (wasm32)	24s
cargo check (msrv) (windows-latest)	2m 16s
cargo check (aarch64-android)	37s
cargo check (msrv) (macos-latest)	42s
cargo doc	1m 46s
Prime LFS Cache	4s
cargo check (msrv) (ubuntu-latest)	53s
cargo test (wasm32)	45s
typos	5s
cargo test (windows-latest)	4m 30s
cargo test (macos-latest)	2m 57s
cargo test (ubuntu-latest)	4m 40s
	29m 48s

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