Improve the performance of 'string enums' #1975

Job	Run time
build_examples	3m 56s
build_benchmarks	53s
Check	1m 2s
build_nightly	1m 43s
Verify that web-sys is compiled correctly	58s
Clippy	7m 2s
dist_linux_aarch64_gnu	2m 46s
dist_linux_x86_64_musl	2m 20s
dist_macos_aarch64	1m 48s
dist_macos_x86_64	3m 20s
dist_windows	2m 57s
doc_api	1m 10s
doc_book	4s
Rustfmt	12s
Build and test the deno example	1m 28s
Run js-sys crate tests	2m 22s
Run native tests	1m 41s
Run wasm-bindgen crate tests with multithreading enabled	1m 47s
Test TypeScript output of wasm-bindgen	1m 16s
Run UI compile-fail tests	41s
Run wasm-bindgen crate tests (unix)	6m 29s
Run web-sys crate tests	6m 53s
Run wasm-bindgen-webidl crate tests	2m 6s
test_examples	1m 52s
deploy	29s
	57m 15s

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