Handle explicit cast of union member

[lucic71]

Union members are translated using accessors:

union u { int a; }
*(char *)&u.a = 0xFF;

becomes

struct u { __bytes: Value<Box<[u8]>> }
impl u { pub fn a() -> Ptr<i32> { ... } } // the accessor

u.a().reinterpret_cast<u8>().write(0xFF) // Before this PR it did not compile: (u.a() as Ptr<u8>).write(0xFF)

This PR replaces the wrong (u.a() as Ptr<u8>) pattern with u.a().reinterpret_cast<u8>()

[lucic71]

This has no effect on the existing union tests now, but it's a step forward to unblock them.