* Intending to use in a new Row type to check types at runtime for
dynamic queries and to guard against schema changes
* Hoping the query! macro can utilize this to allow accepting N
rust types for 1 sql type and returning N rust types for 1 sql
type.
- 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`
