Fixed parsing of custom derivation attributes.
Fixed parsing of custom derivation attributes.

It was incorrectly requiring some parts to be *just* idents without
arguments, which was breaking with multiple custom derivations.

Also simplified the derive attribute splitting a little by removing the
superfluous `#[..]` wrapping.

TaborKelly/rust-custom-derive,durka/rust-custom-derive,durka/rust-custom-derive,TaborKelly/rust-custom-derive