Fix strict-aliasing violation on traits holding inner fields

Fix strict-aliasing violation on traits holding inner fields

When we map a trait method which returns a reference to a struct,
we map it by storing said struct in the trait implementation
struct. Then, we create a setter method which ensures the new field
is set. Sadly, because the original Rust trait method may take a
non-mutable reference to self, we have to update the field without
a mutable reference to the trait implementation struct.

Previously we did this by simply unsafe-casting a pointer to
mutable, which violates the aliasing rules in Rust. In a recent
rustc (at least on macOS), this broke.

Here, we convert the stored struct to wrap it in an `UnsafeCell`,
which fixes the issue.