Bump `walrus` to v0.20 #1542

Job	Run time
Rustfmt	10s
Run wasm-bindgen crate tests (unix)	9m 10s
Clippy	6m 52s
Run js-sys crate tests	1m 55s
Verify that web-sys is compiled correctly	1m 15s
Check	57s
Run native tests	1m 38s
Run web-sys crate tests	3m 52s
Run wasm-bindgen crate tests with multithreading enabled	4m 26s
Run wasm-bindgen-webidl crate tests	1m 59s
build_benchmarks	1m 31s
build_examples	3m 24s
build_nightly	2m 46s
dist_linux_aarch64_gnu	4m 52s
dist_linux_x86_64_musl	2m 23s
dist_macos_aarch64	2m 49s
dist_macos_x86_64	2m 49s
doc_api	1m 8s
dist_windows	6m 7s
doc_book	11s
Build and test the deno example	1m 26s
Test TypeScript output of wasm-bindgen	2m 0s
Run UI compile-fail tests	6m 3s
test_examples	4m 36s
deploy	1m 21s
	1h 15m 40s

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