Prior to this commit, the Rust SDK incorrectly encoded a `Token` passed to `connect`,
making it impossible to re-connect an existing user.
(Shows what I get for never testing that.)
With this commit, auth tokens are correctly base64-encoded
in an `Authorization: Basic` header with the username `token`,
which allows re-connecting as an existing user.

Also, it was difficult to re-use `Credentials` from the Rust SDK, as:
- they were opaque types with non-exported members
- they did not implement any serialization / deserialization
This commit makes `token.string` and `identity.bytes` public fields,
and implements SATN `Serialize` and `Deserialize`
for `Identity`, `Token` and `Credentials`,
allowing clients to save their credentials for re-use, e.g. to a file.

- `subscribe_owned` accepts `Vec<String>`; behaves like `subscribe`.
  - While developing `letrs`, a demo game, we needed to generate query strings at runtime,
    and passing them to `subscribe` would have involved an unergonomic dance
    of taking references which would then be converted to owned containers
    by the SDK.
    `subscribe_owned` eliminates that, allowing client authors
    to construct a set of queries at runtime and pass it directly to the SDK,
    without intervening referencies and copies.
- Docstrings for `subscribe` and `subscribe_owned`.
- Methods on `Identity` and `Token` for converting to/from strings and byte-vectors.
- Removed long-forgotten `println` within `connect`
  which wrote the computed URI to stdout.