Add Writeable getters for some types, use in FFI

[Manishearth]

I didn't do all of the APIs, just some of them.

I wasn't sure if I should make IxdtfStringBuilder writeable, or give it a .writeable() method that returns an impl Writeable (the inner type). Open to opinions.

[nekevss]

Small nit. The other is more of a general thought; I'll leave the action up to you.

[nekevss]

thought (non-blocking): this should be moved to an ixdtf module or some other module.

The initial thought was to try and move this functionality up into ixdtf. Still not entirely sure whether that should be done, but I don't think it should live in the parsers mod. I think use temporal_rs::ixdtf::IxdtfStringBuilder makes more sense than use temporal_rs::parsers::IxdtfStringBuilder.

This can be done in a separate PR though. Also, open to other suggestions on where IxdtfStringBuilder should live.

[Manishearth]

I think this makes a lot of sense to live in the ixdtf crate. A module also seems fine.

[Manishearth]

I filled in the rest of the types. I couldn't do Duration because it doesn't use IxdtfStringBuilder: someone should write a separate Writeable impl for that. I also didn't do ZonedDateTime due to #330; IxdtfStringBuilder needs to borrow from the timezone. It could also be written to use a type that is effectively (time_zone, IxdtfStringBuilder).

[nekevss]

LGTM