fix: add missing feature flags

[tomkarw]

Closes https://github.com/hyperium/hyper-util/issues/7.

This PR:

• adds missing feature flags that were referenced in code, but not in Cargo.toml: runtime, tcp, http1, http2
• copies Never struct from hyper and uses it in client module to make it compile when features are enabled
• fixes 3 Clippy warnings about preferring Copy to Clone

There is one test still failing: client::connect::http::tests::client_happy_eyeballs. It's a bit over my head, so I didn't attempt to fix it.

[tomkarw]

The fact that the all checks pass, even though there is a failing test, is worrying. It's also the reason code that doesn't compile was merged.

I'll make cargo check and cargo test use --all-features flag.
The project already includes something called cargo-hack, but it doesn't seem to serve its purpose.

I'll wait for @seanmonstar for guidance.