While attempting to reproduce rust-lang/rust#47086 I noticed the
following warning:

```shell
> rustc /dev/null --crate-type proc-macro
warning: unused variable: `registrar`
 --> /dev/null:0:1
```

As there are no macros to register the automatically generated registrar
function for the crate has no body. As a result its `registrar` argument
is unused triggering the above warning.

The warning is confusing and not easily actionable by the developer. It
could also be triggered legitimately by e.g. having all of the macros in
a crate #[cfg]'ed out.

Fix by naming the generated argument `_registrar` inside
`mk_registrar()`. This suppresses the unused variable warning.