stop to strip 'impl' from impl trait type alias

[rchaser53]

fix: #3815

[topecongiro]

IMHO we should handle whether to add impl by adding a wrapper GenericBounds and implement Rewrite trait against it:

struct GenericBounds<'a> {
    generic_bounds: &'a ast::GenericBounds,
    has_impl: bool,
}

impl<'a> Rewrite for GenericBounds<'a> {
    fn rewrite(&self, context: &RewriteContext<'_>, shape: Shape) -> Option<String> {
        self.generic_bounds.rewrite(context, shape)
            .map(|s| if self.has_impl { format!("impl {}", s } else { s })
    }
}

and use it inside rewrite_type_alias and rewrite_opaque_type.

[rchaser53]

I added the commit to remove has_impl in GenericBounds. I don't think it necessary.

[topecongiro]

Good point, thanks for the update! The code looks good to me.

[topecongiro]

Thank you!