Implement header/struct flattening

[PietroGhg]

BMv2 (and potentially other targets) need structs and headers to be flattened so that they don't contain other aggregate types. This is implemented in p4c by several passes, such as FlattenHeaders, FlattenInterfaceStructs and NestedStructs

[asl]

I think we can have this almost for free via mainline SROA, we almost have the required interfaces ready :)

[PietroGhg]

I'm picking up this task