Export feature added: "splatting" structs into multiple columns

We’ve added a convenience which allows groups of annotations to be exported more easily. In all export commands, you can now use the struct.* syntax to export all fields of a struct as separate columns. See that in the examples for export_variants here:

https://hail.is/hail/hail.VariantDataset.html#hail.VariantDataset.export_variants