Make X509Store shareable between contexts

[kornelski]

#362

[nox]

I don't like that you introduced a panic in cert_store_mut, as it can make currently existing code panic now when combined with set_cert_store. Let's wait for boring 5.

[kornelski]

Existing code won't panic, because I haven't changed the semantics of set_cert_store.

It think it's better to deprecate set_cert_store first in v4, and then make a breaking behavior change in v5. The current API will force users to decide if they want immutable or mutable store by switching to one of the new methods.

[nox]

Existing code won't panic, because I haven't changed the semantics of set_cert_store.

Existing code can do .set_cert_store(store) and then .cert_store_mut().

Never mind.

[nox]

I assumed that the PR also implemented Clone for X509Store hence my confusion, I'm ok with the changes now.

[nox]

This is clever btw, nice of you to find a way without having to do boring 5.

[0x676e67]

It looks good, so it can also be referenced in the external Arc packaging Cert Store.