Make iana-time-zone optional #892

Job	Run time
Documentation	26s
Lint	21s
MSRV	24s
Test feature matrix	2m 47s
Test FFI (stable, ubuntu-latest)	15s
Test (stable, windows-latest)	2m 35s
Test (stable, macos-latest)	26s
Test (stable, ubuntu-latest)	32s
	7m 46s

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