Ensure braids implement StructuralEq and StructuralPartialEq

[neoeinstein]

This will allow for using const values in pattern matching when that lands, as in the following:

#[braid]
struct MyData;

const TEST_VAL: &MyDataRef = MyDataRef::from_str("test");

match some_data_ref {
    TEST_VAL => println!("test!"),
    _ => println!("not a test…"),
}

[neoeinstein]

Still working toward this. Even with const values, the compiler doesn't like this custom type as the pattern to assert on:

cannot use unsized non-slice type `Type` in constant patterns

See rust-lang/rust#87046

[neoeinstein]

This is closed via #13, but the compiler still prevents this as mentioned above.