Bump `walrus` to v0.20 (#3483) #1543

Job	Run time
Rustfmt	7s
Run wasm-bindgen crate tests (unix)	11m 56s
Clippy	5m 11s
Verify that web-sys is compiled correctly	1m 18s
Check	1m 33s
Run js-sys crate tests	2m 7s
Run native tests	2m 40s
Run wasm-bindgen crate tests with multithreading enabled	2m 32s
Run web-sys crate tests	6m 7s
Run wasm-bindgen-webidl crate tests	1m 58s
build_benchmarks	1m 36s
build_examples	3m 30s
build_nightly	1m 33s
dist_linux_aarch64_gnu	5m 23s
dist_linux_x86_64_musl	2m 22s
dist_macos_aarch64	1m 58s
dist_macos_x86_64	2m 18s
dist_windows	4m 12s
doc_book	18s
Build and test the deno example	48s
Test TypeScript output of wasm-bindgen	57s
Run UI compile-fail tests	4m 12s
doc_api	1m 13s
test_examples	3m 27s
deploy	1m 11s
	1h 10m 27s

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