export Bytes type through DataRowBody::storage_bytes method.

[fakeshadow]

Exposing Bytes type can be beneficial as #996 suggested.

Regarding high level usage it can be introduced as following pesudo api:

impl Row {
    fn get2<I, T>(&self, idx: &I) -> Result<T, Error>
    where
        I: RowIndex + fmt::Display,
        T: FromSqlOwned,
    {
        ...
    }
}

trait FromSqlOwned {
  fn from_sql(ty: &Type, raw: &Bytes) -> Result<Self, Box<dyn Error + Sync + Send>>;
}

// new type of Bytes with utf8 encoding check for cheap clone.
struct ByteString(Bytes);

impl FromSqlOwned for ByteString {
  fn from_sql(ty: &Type, raw: &Bytes) -> Result<Self, Box<dyn Error + Sync + Send>> {
    std::str::from_utf8(raw.as_ref())?
    ByteString(raw.clone())  
  }
}

Closes #996

[fakeshadow]

Regarding changelog should I add an unreleased session on top?

[sfackler]

Nope, no need.

[ParkMyCar]

I see the fn buffer_bytes(...) method now exists on DataRowBody, has anything like the high level interface been implemented yet? Given a Row I don't believe you can access the DataRowBody given it's a private field and looking through the docs there don't appear to be any accessors.

My goal is to get zero-copy accessing of a bytea column, similar to what's mentioned in #996. More than happy to submit a PR if you'd be open to something like the high level usage proposed here @sfackler?