Use a temporary private fork of tokio-rustls that uses Rustls 0.12
until tokio-rustls 0.4 is released.
This upgrades, among other things, *ring* to 0.12, which will ensure
that it still builds in the Rust 1.20 release coming this week even if
backward-compatibility-breaking changes to rustc aren't fixed before
the release.
With this change, h2 can build and run without any manual configuration steps for -msvc targets. Previously manual installation of OpenSSL libraries was required.
