Reject multiple top-level attributes

[michaelsproul]

This compiles while ignoring the second attribute:

#[superstruct(variants(A, B))]
#[superstruct(variants(C, D))]
struct MyThing;

We should reject it, but it's not clear how to do this. See https://github.com/sigp/superstruct/pull/38 and the TODO added there.