Make remaining methods const #2841

Job	Run time
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	19s
check-docs	23s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	9s
cross-targets (x86_64-sun-solaris)	2m 23s
alternative_targets (ubuntu-latest, aarch64-linux-android)	12s
cross-tests (ubuntu-latest)	1m 50s
features_check (ubuntu-latest)	5m 29s
features_check (macos-latest)	10m 26s
no_std (ubuntu-latest, thumbv6m-none-eabi)	19s
rust_msrv (ubuntu-latest)	56s
features_check (windows-latest)	9m 1s
rust_versions (ubuntu-latest, stable)	1m 5s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	14s
rust_versions (ubuntu-latest, beta)	1m 2s
rust_versions (ubuntu-latest, nightly)	1m 6s
test_wasi (ubuntu-latest, wasm32-wasi)	22s
test_wasm (ubuntu-latest)	22s
timezones (ubuntu-latest, ACST-9:30)	1m 17s
timezones (ubuntu-latest, EST4)	1m 26s
timezones (ubuntu-latest, UTC0)	1m 18s
timezones (ubuntu-latest, Asia/Katmandu)	1m 15s
timezones (macos-latest, ACST-9:30)	1m 24s
timezones (macos-latest, EST4)	1m 37s
timezones (macos-latest, UTC0)	45s
timezones (macos-latest, Asia/Katmandu)	1m 25s
timezones (windows-latest, ACST-9:30)	2m 16s
timezones (windows-latest, EST4)	2m 8s
timezones (windows-latest, UTC0)	2m 26s
timezones (windows-latest, Asia/Katmandu)	2m 3s
	54m 58s

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