Add an option to preserve the original field order

[joroKr21]

I was experimenting with deriving a visitor for models generated by Typify with derive_visitor but it sorts the struct fields unconditionally which changes the visit order. Ideally there should be an option in TypeSpaceSettings to disable the sorting.

[ahl]

Good idea! We (currently) use schemars 0.8 and its structures into which we deserialize json schemas; it has a preserve_order feature flag. There are some edge cases to consider such as what to do if we "merge" two objects e.g. allOf [ { type: object, .. }, { type: object, .. } ].