Add Shared type to fix inference fail on nightly

[KodrAus]

Hi there!

In rust-lang/rust#42565 some new impls were added to Arc and friends that are breaking type inference on this crate for string keys/values. This was originally reported here.

The trait bounds here worked previously because we only had impl<T> From<T> for Arc<T> in the standard library, so in the bound Arc<T>: From<U>, U could only ever be T. That's not the case anymore, so Rust can't figure out what T should be from U. I'm assuming the reason for these bounds here was so that you could pass either T or Arc<T> to some of the datastructures and have it do the right thing.

To work around this I've added a trait that's implemented only for T and Arc<T>, so when we get a generic T: Shared<U> we know that T must be either U or Arc<U>. I haven't tried to come up with a good name for this trait or add any docs, in case this isn't the way you want to solve this problem.

This is technically a breaking change. An alternative solution is that callers will just need to give Rust a hand and specify the type of maps and things when it can't figure it out itself.

cc: @bodil

[bodil]

Made a new major release with this.