Remove conservative_impl_trait feature

[cramertj]

This is needed to land the stabilization of impl Trait

cc rust-lang/rust#49255 (comment)

r? @oli-obk

[solson]

As @oli-obk mentioned on the other thread, Miri's build has been broken for a while anyway so this shouldn't block the stabilization, but thanks for the PR. :)

I can't actually merge it without skipping our required CI build, so we'll need to wait on #364 anyway (or merge this into that).

[cramertj]

Yup, sorry-- this isn't how we'd do this anyway (it'd be better to allow(stable_features) so that it builds with and without the feature)