Rust: Flow through variants

[hvitved]

This PR adds support for tracking data flow through construction and deconstruction of enum variants. For example, we can now identify flow in cases such as

enum MyEnum {
    A(i64),
    B(i64),
}

let s1 = MyEnum::A(source());
match s1 {
    MyEnum::A(n) => sink(n), // flow
    MyEnum::B(n) => sink(n), // no flow
}

Commit-by-commit review is suggested.