#1543 implemented a restricted form of Rust union support, in which a union can be interpreted only at the same type (i.e. Crucible representation) with which it was initialized. This is useful, but incomplete. To better mimic Rust, we'll need to support reinterpretation of union values at more types.

#1543 introduced a couple of xfail tests that involve union memory reinterpretation and that (I believe) represent well-defined Rust, so a solution to this issue ought to cause them to pass.