Impl serde::Serialize and serde::Deserialize for TimeDelta #3451

Job	Run time
features_check (ubuntu-latest)	3m 53s
no_std (ubuntu-latest, thumbv6m-none-eabi)	19s
rust_msrv (ubuntu-latest)	51s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	14s
rust_versions (ubuntu-latest, stable)	1m 14s
rust_versions (ubuntu-latest, beta)	1m 15s
timezones (ubuntu-latest, ACST-9:30)	2m 5s
timezones (ubuntu-latest, EST4)	1m 25s
timezones (ubuntu-latest, UTC0)	1m 29s
timezones (ubuntu-latest, Asia/Katmandu)	1m 33s
timezones (macos-latest, ACST-9:30)	36s
timezones (macos-latest, EST4)	41s
timezones (macos-latest, UTC0)	40s
timezones (macos-latest, Asia/Katmandu)	36s
timezones (windows-latest, ACST-9:30)	2m 53s
timezones (windows-latest, EST4)	2m 11s
timezones (windows-latest, UTC0)	2m 45s
timezones (windows-latest, Asia/Katmandu)	4m 19s
rust_versions (ubuntu-latest, nightly)	1m 6s
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	20s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	15s
alternative_targets (ubuntu-latest, aarch64-linux-android)	27s
check-docs	22s
cross-targets (x86_64-unknown-illumos)	1m 19s
cross-tests (ubuntu-latest)	1m 51s
test_wasi (ubuntu-latest, wasm32-wasi)	39s
test_wasm (ubuntu-latest)	1m 26s
	36m 44s

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