[Safety] UnionBuilder Doesn't Check Types

[tustvold]

Describe the bug

The following code passes, despite transmuting memory in undefined ways

let mut builder = UnionBuilder::new_sparse(2);
builder.append::<Float32Type>("a", 1.0).unwrap();
builder.append::<Int32Type>("a", 1).unwrap();

let array = builder.build().unwrap();

To Reproduce

Test above

Expected behavior

UnionBuilder::append should error if the types don't match

Additional context
Add any other context about the problem here.