/// Create a SQL expression with `AnyMysqlType` as the value type.
///
/// Scalar arguments must implement `Into<AnyMysqlType>` -- supported types are:
/// `i8`-`u32`, `f32`/`f64`, `bool`, `String`, and `&str`.
///
/// ```ignore
/// let expr = mysql_expr!("SELECT * FROM product WHERE price > {}", 100i64);
/// let result = db.execute(&expr).await?;
/// ```