Add from_partial for YearMonth/MonthDay FFI

[Manishearth]

No description provided.

[nekevss]

Just to confirm, have you seen #342? That's going to be a breaking change here to YearMonth::from_partial

[Manishearth]

I have not, that is also an acceptable API. Still need it for MonthDay though. Happy to wait, I have an acceptable workaround for now.

[nekevss]

I think I'd prefer to wait on this for now. Because there's also #349 to consider too.

[Manishearth]

I'm actually finding myself needing this: the current API is insufficient to implement ToTemporalYearMonth, which needs a way to pass down monthCode. The current hacky implementation I have fails tests.

[nekevss]

Sounds good. I think this really just needs to be updated to the more recent changes.

[Manishearth]

Oh, wait, I didn't realize #342 landed. I should just use that lol.

[Manishearth]

Ah, I still need it over FFI.

[Manishearth]

I'll keep this PR open until I can fix that.

[Manishearth]

@nekevss I updated this to use the new PartialYearMonth.

Over FFI I still use PartialDate for both of these. I could add a PartialYearMonth over FFI but I don't actually have the need to have nitty gritty Partial types here.

MonthDay must use PartialDate since for non-ISO calendars you can set the year. Misread the spec, see #412