Add type-specific write methods to the StructWriter trait

[zslayton]

pub trait StructWriter {
    /// Writes a struct field using the provided name/value pair.
    fn write<A: AsRawSymbolTokenRef, V: WriteAsIon>(
        &mut self,
        name: A,
        value: V,
    ) -> IonResult<&mut Self>;
   
     // TODO: 
     // fn write_null<A: AsRawSymbolTokenRef>(&mut self, name: A, value: IonType) -> IonResult<()>;
     // fn write_bool<A: AsRawSymbolTokenRef>(&mut self, name: A, value: bool) -> IonResult<()>;
     // ...etc.
}