Gateway: Simplify return value of join/join_gateway (#157)

Replaces the annoying dual-return (i.e., created `Call` *and* `Result<x>`) with a single `Return<Call/ConnectionInfo>`. Users are now informed via that a `Call` is created -- thus, cleanup in event of connection failure is now their responsibility.

Tested using `cargo make ready`.

Closes #65.

src/input/adapters/cached/error.rs

@@ -91,8 +91,7 @@ impl Display for CodecCacheError {
             Self::Parse(p) => f.write_fmt(format_args!("failed to parse audio format: {p}")),
             Self::Opus(o) => f.write_fmt(format_args!("failed to create Opus encoder: {o}")),
             Self::MetadataEncoding(m) => f.write_fmt(format_args!(
-                "failed to convert track metadata to JSON: {}",
-                m
+                "failed to convert track metadata to JSON: {m}"
             )),
             Self::MetadataTooLarge => f.write_str("track metadata was too large, >= 32kiB"),
             Self::CreatePanicked => f.write_str("sync thread panicked while creating stream"),