The `define_extension_trait` macro originally didn't support `#[inline]`
or other attributes for simplicity. We still want them so this commit
adds basic support for it. It adds the `doc` attributes to trait
*definition* only and adds all other attributes to the *implementation*
only. This should support `#[inline]` and other attributes. The downside
is it doesn't support adding non-doc attributes to trait *definition*
but I can't think of any relevant ones that we would want and we can
find a solution later if we do.
fcc3cb03f0