impl Deref for CompleteStr, CompleteByteSlice

[lorepozo]

With the impending increased use of CompleteStr and CompleteByteSlice in 4.0, this should give users (myself included) an easier time with low overhead.

Instead of:

named!(var<CompleteStr, Item>,
        do_parse!(
            name: ws!( alpha ) >>
            (Item::Var(name.0.to_string()))
        ));

We can avoid explicitly getting name.0 thanks to Deref:

named!(var<CompleteStr, Item>,
        do_parse!(
            name: ws!( alpha ) >>
            (Item::Var(name.to_string()))
        ));

[Geal]

Thanks!