Support compact encoding for tuple enum items

[gavofyork]

This builds:

pub enum Example {
  Item { #[codec(compact)] x: u32 },
}

Yet this doesn't:

pub enum Example {
  Item ( #[codec(compact)] u32 ),
}

[gui1117]

It should work already

parity-scale-codec/tests/mod.rs

Lines 100 to 107 in f0341da

	#[derive(Debug, PartialEq, Encode, Decode)]
	enum TestCompactAttributeEnum {
	Unnamed(#[codec(compact)] u64),
	Named {
	#[codec(compact)]
	bar: u64,
	},
	}

I tried the example you provided and it worked to me.

[gui1117]

tests cover the case and tests that it is indeed compact. thus I think we can close it