Enable logging in rustls and tracing-using dependencies

[blyxxyz]

Enable rustls's logging feature to start emitting logs.

Enable the tracing crate's log feature to hook up the dependencies that log via that crate.

hyper can use tracing but it's currently unstable and locked behind RUSTFLAGS='--cfg hyper_unstable_tracing' so we shouldn't use it yet.

This partially addresses #389.

$ RUST_LOG=trace/ALPN xh https://example.org
[0.495665s DEBUG rustls::client::hs] ALPN protocol is Some(b"h2")
[0.499526s TRACE hyper_util::client::legacy::client] ALPN negotiated h2, updating pool
HTTP/2.0 200 OK
[...]
$ RUST_LOG=rustls xh https://example.org
[0.288085s DEBUG rustls::client::hs] No cached session for DnsName("example.org")
[0.288657s DEBUG rustls::client::hs] Not resuming any session
[0.288767s TRACE rustls::client::hs] Sending ClientHello Message {
    version: TLSv1_0,
    payload: Handshake {
[...]
[0.698465s DEBUG rustls::client::hs] Using ciphersuite TLS13_AES_256_GCM_SHA384
[0.698508s DEBUG rustls::client::tls13] Not resuming
[0.698530s TRACE rustls::client::client_conn] EarlyData rejected
[0.699267s DEBUG rustls::client::tls13] TLS1.3 encrypted extensions: [Protocols([ProtocolName(6832)])]
[0.699342s DEBUG rustls::client::hs] ALPN protocol is Some(b"h2")
[0.699578s TRACE rustls::client::tls13] Server cert is
CertificateChain([CertificateDer(0x3082076e3082[...]

native-tls barely has any logging so we don't get much useful info from there yet.

---

Additionally, print errors even if logs are getting filtered. Before:

$ RUST_LOG=foobar xh ssh://example.org
[no output]

After:

$ RUST_LOG=foobar xh ssh://example.org
xh: error: builder error for url (ssh://example.org)

Caused by:
    URL scheme is not allowed

This got confusing when debugging.