lang reprs

Lang reprs abstractly describe an interop target for the layout of structs and enums and the like. These currently exactly match the "lang reprs" in KDLScript.

For each test we will generate a copy of it for every enabled lang repr (changing the definitions of all types which don't specify an explicit repr).

--reprs

All of the following reprs are enabled by default, and only these reprs are supported.

• c: layout structs in a C-compatible way (repr(C))
• rust: layout structs in a Rust-compatible way (repr(Rust))