Macro tree_buf::impl_primitive_reader_writer

macro_rules! impl_primitive_reader_writer {
    ($T:ty) => { ... };
}

This is a substitute for a blanket impl, since the blanket impl makes us run afoul of orphan rules. The core problem is that if we want to impl<'a, T: Primitive> Writable for PrimitiveBuffer, then we can't also impl Writer for Box