Macro diesel::operator_allowed

macro_rules! operator_allowed {
    ($tpe:ty, $op:ident, $fn_name:ident) => { ... };
}

Implements the Rust operator for a given type. If you create a new SQL function, which returns a type that you'd like to use an operator on, you should invoke this macro. Unfortunately, Rust disallows us from automatically implementing Add and other traits from std::ops, under its orphan rules.