Tracking issue for lunar calendars

[Manishearth]

When we implement lunar calendars (Hebrew, Chinese, etc) there will be a bunch of areas of the code we'll have to update.

This issue serves to track those dependencies.

Some known dependencies:

• The solution for https://github.com/unicode-org/icu4x/issues/2052 will need to be updated to handle the fact that month lengths change throughout the year
• The code for month code datagen (that i am currently working on, no PR yet) is much simpler and will need to be calendar specific https://github.com/unicode-org/icu4x/issues/486 . In particular, the hebrew and chinese calendars handle leap month symbols differently (hebrew has an extra month, chinese has leap month patterns)
• ...?

cc @pandusonu2 , though most of this you won't have to worry about for the MVP of any of these calendars

[Manishearth]

There's an existing impl of the Persian calendar in #1626 that can be adapted