Init persian calendar

[HKalbasi]

This PR implements converting Persian calendar to Iso and inverse. offset_date() and until() are blocked on #1470

There is also a minor fix for Iso::iso_from_fixed.

[Manishearth]

https://github.com/unicode-org/icu4x/pull/1614 has merged

[jira-pull-request-webhook[bot]]

Notice: the branch changed across the force-push!

• components/calendar/src/calendar_arithmetic.rs is now changed in the branch
• components/calendar/src/lib.rs is different
• components/calendar/src/persian.rs is different

View Diff Across Force-Push

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[sffc]

Closing this PR due to inactivity, but feel free to re-open if you pick this up again! @Manishearth will post some more pointers.