Add Month::num_days() #3526

Job	Run time
check-docs	22s
alternative_targets (ubuntu-latest, wasm32-unknown-emscripten)	10s
cross-targets (x86_64-unknown-illumos)	1m 8s
alternative_targets (ubuntu-latest, aarch64-apple-ios)	21s
cross-tests (ubuntu-latest)	1m 58s
alternative_targets (ubuntu-latest, aarch64-linux-android)	11s
features_check (ubuntu-latest)	3m 37s
no_std (ubuntu-latest, thumbv6m-none-eabi)	12s
rust_msrv (ubuntu-latest)	2m 8s
no_std (ubuntu-latest, x86_64-fortanix-unknown-sgx)	12s
rust_versions (ubuntu-latest, stable)	1m 8s
rust_versions (ubuntu-latest, beta)	1m 12s
test_wasi (ubuntu-latest, wasm32-wasi)	32s
rust_versions (ubuntu-latest, nightly)	1m 1s
test_wasm (ubuntu-latest)	36s
timezones (ubuntu-latest, ACST-9:30)	1m 20s
timezones (ubuntu-latest, EST4)	1m 17s
timezones (ubuntu-latest, UTC0)	1m 16s
timezones (ubuntu-latest, Asia/Katmandu)	1m 15s
timezones (macos-latest, ACST-9:30)	43s
timezones (macos-latest, EST4)	43s
timezones (macos-latest, UTC0)	45s
timezones (macos-latest, Asia/Katmandu)	42s
timezones (windows-latest, ACST-9:30)	3m 35s
timezones (windows-latest, EST4)	2m 15s
timezones (windows-latest, UTC0)	2m 51s
timezones (windows-latest, Asia/Katmandu)	3m 1s
	34m 31s

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