Macro singleton 

macro_rules! singleton {
    ($name:ident, $getter:ident, $ty:ident) => { ... };
}

Create a singleton and accompanying static variable for a LogicalTypeRef of a NativeType.

• name: name of the static variable, must be unique.
• getter: name of the public function that will return the singleton instance of the static variable.
• ty: the NativeType.