* Allow `conn.fetch(" ... ")` to be called where `conn` is an owned Connection
* Executor::fetch -> RefExecutor::fetch_by_ref
* Executor::fetch_by_ref -> Executor::fetch
* Move `Connection::describe` to `Executor::describe`
* `Transaction` is no longer a `Connection`
* `Connection` has `Executor` as a super-trait again which greatly simplifies bounds
* fix `Pool` to reduce possibility of "leaking" connections
now uses RAII guards to control `SharedPool::size`
* add smoke test for `Pool` to both Postgres and MySQL tests
add `Pool::is_closed()`
* fix documentation re: pool
* refactor pool implementation to not use futures oneshot channels
https://github.com/launchbadge/sqlx/pull/84#issuecomment-580476223
* run cargo fmt
* Pool: remove superfluous guard struct, document some internal methods
- Remove `hex` from root `Cargo.toml`
- Make `hmac` crate optional
- Clean up checking mechanisms for "SCRAM-SHA-256"
- Use `str::from_utf8` instead of `String::from_utf8_lossyf
- Update `Sasl*Response` structs be tuple structs
- Factor out `len` in `SaslInitialResponse.encode()`
- Use `protocol_err` instead of `expect` when constructing `Hmacf
instances
- Remove `it_connects_to_database_user` test as it was too fragile
- Move `sasl_auth` function into `postgres/connection` as it more
related to `Connection` rather than `protocl`
- Return an error when decoding base64 salt rather than panicing
in `Authentication::SaslContinue`
