Allow ordering of fields in a record

[blackwinter]

No idea if Catmandu provides this, but it would be awesome if we could control the order in which fields are emitted. Both by adding/moving fields in relation to others and by order()ing the record as a whole. This would help with consistency, especially in tests.

[TobiasNx]

@blackwinter hinted to a workaround for now. One has to emit the elements firstin a certain order e.g. as empty fields with add_field.