Keep struct fields order in schema

Struct fields are stored in a BTreeMap in schema properties, this sorts them alphabetically.

But, often, at least in my projects, struct fields order has some semantic meaning (e.g. in one project I have a Span struct with start and end fields, and others; it makes sense to have those two fields one after the other, and in this order).

I think it would make more sense in general to keep the fields order (if you want them alphabetically sorted, you'll probably sort them in your struct anyway?).