Update tokio-tungstenite to 0.15

[Erk-]

This is a revive of #1028 as the parts for #1058 are yet to be published to crates.io.

rescinded

[Gelbpunkt]

This is a wrap-up of discussions and research from the #dev channel on Discord:

• tokio-tungstenite with rustls 0.15 depends on tungstenite 0.14's rustls feature, which pulls in rustls-native-certs dependencies, namingly: bitflags, core-foundation, core-foundation-sys, openssl-probe, rustls-native-certs, schannel, security-framework and security-framework-sys
• These dependencies are not used, thus tokio-tungstenite 0.15 works in a webpki-dependent environment, but compile times and dependency bloat are increased
• This has been fixed in a later release of tungstenite, 0.15
• tokio-tungstenite's latest git tree depends on this new version and allows for choosing between webpki-roots and native-roots - which previously was not possible
• It also removes a dependency on pin-project, which can significantly improve compile times

The only real upside in bumping tokio-tungstenite to 0.15 is a faster input buffer, but the impact of that might be negligible because network latency is the bottleneck here. It is probably not worth the dependency mess we'd put ourselves in by bumping it.

Rather, we should wait for 0.16 to release to allow fine-grained rustls feature flags on gateway and avoid this TLS dependency mess. This also can be done at any time, because tokio-tungstenite is not part of twilight's public API.