Client refactor

[Gelbpunkt]

Hi Tim,

I've done the following things:

• lint the whole project against clippy's pedantic level and fix the issues (a couple #[must_use], missing error sections in docs, possible truncations or redundant else blocks)
• ~~added a rustfmt config option that seems to match your import granularity~~ nightly only
• removed the rust toolchain files (LMK if this needs to be readded)
• disabling all TLS is now possible (fixes #220)
• added support for Rustls as a pure rust TLS backend
• removed support for OpenSSL because it is the backend for native-tls on Unix platforms anyways
• reworked TLS to use a Connector struct that can be reused via ClientBuilder::set_connector to save memory when creating multiple clients
• added a small abstraction around WebSocket close codes
• replaced futures with futures-util where possible
• disabled default features on big dependencies where possible
• and probably more, please make sure to check the changes

After reading your reply in the email, I've also readded the sync support that I initially removed.

I'm happy to address any comments or nits you find with the changes!

Since CI doesn't run on forks, I hope I didn't break too much :)

If you decide to merge this, please squash the changes to not make it 25 individual commits.

Signed-off-by: Jens Reidel [email protected]

[Gelbpunkt]

There are a couple of things here that I have a weird feeling about, I'll comment on the specific parts

[brandonros]

Is it possible for this to land? rustls would be pretty cool / avoid openssl-sys cross compiling weirdness.

[1tgr]

Superseded by #327.