Add language tests for aggregate types
This adds some tests for struct and union types, ensuring that they satisfy the rules for all structs and unions - namely that fields of structs do not overlap, fields are well-aligned, and the size of the entire.
The reference annotations used are from https://github.com/rust-lang/reference/pull/1654, though the rules tested here were FCPed in <https://github.com/rust-lang/reference/pull/1152>.