Flattening a unit struct should work like flattening unit itself

[jonhoo]

#1874 landed support for flattening the unit type after a report in #1873. However, it did not override deserialize_unit_struct and serialize_unit_struct, so while flattening () now works, flattening struct Unit; does not. It's not clear whether this was intentional or not, though it seems reasonable to support unit structs the same as unit here.

[jonhoo]

If uncontroversial, I've put up a simple implementation following the footsteps of #1874 in #2802.