message: remove as_string
diff: @@ -772,17 +772,12 @@ pub trait ToSql: fmt::Debug {
  /// Specify the encode format
  fn encode_format(&self) -> i16 { 1 }
  -
  - /// return string representation
  - fn as_string(&self) -> String {
  - panic!("as_string not implemented for {:?}", self)
  - }
  }
  /// A Wrapper type used for sending query parameters encoded as unknown.
  #[derive(Debug)]
  -pub struct Unknown<'a>(pub &'a (dyn ToSql + Sync));
  +pub struct Unknown<'a>(pub &'a str);
  impl ToSql for Unknown<'_> {
  fn to_sql(
  @@ -792,7 +787,7 @@ impl ToSql for Unknown<'_> {
  ) -> Result<IsNull, Box<dyn Error + Sync + Send>> {
  match *self {
  Unknown(val) => {
  - types::text_to_sql(&val.as_string(), out);
  + types::text_to_sql(val, out);
  Ok(IsNull::No)
  }
  }
  @@ -939,7 +934,7 @@ impl<'a> ToSql for &'a str {
  _ => false,
  }
  }
  - fn as_string(&self) -> String { self.to_string() }
  +
  to_sql_checked!();
  }
  @@ -963,7 +958,7 @@ impl ToSql for String {
  fn accepts(ty: &Type) -> bool {
  <&str as ToSql>::accepts(ty)
  }
  - fn as_string(&self) -> String { self.clone() }
  +
  to_sql_checked!();
  }
  @@ -978,10 +973,6 @@ macro_rules! simple_to {
  Ok(IsNull::No)
  }
  - fn as_string(&self) -> String {
  - format!("{}", &self)
  - }
  -
  accepts!($($expected),+);
  to_sql_checked!();
  