Failure when using num_derive on latest nightly

[est31]

To reproduce, run cargo test on the num/derive subdir, you will get lots of "a type named ... has already been defined in this module".

I can reproduce with 1.15.0-nightly (0ed951993 2016-11-14).

This change in behaviour was probably caused by rust-lang/rust#37614, but its actually wanted behaviour.

Basically the proc macro isn't expected anymore to output the item itself, only the impl's.

[hauleth]

Will investigate.

[est31]

@hauleth any news?

[hauleth]

@est31 #242