Deny derive attributes on enum variants

[wackbyte]

Adds a check to produce an error when an enum variant with an #[arbitrary] derive attribute is encountered.
I did not implement the ability to apply the attribute transparently to single-field variants.

Preview:

error: invalid `arbitrary` attribute. it is unsupported on enum variants. try applying it to a field of the variant instead
 --> src\lib.rs:1675:5
  |
7 |     #[arbitrary(default)]
  |     ^^^^^^^^^^^^^^^^^^^^^

Closes #149 and #150.

[fitzgen]

Thanks!