Improve Debug representations

[dtolnay]

This is ported from rust-lang/rust#49748. I decided not to print a span: field when we are not emulating spans because they are just noise.

TokenStream [
    Group {
        delimiter: Bracket,
        stream: TokenStream [
            Term {
                sym: a
            },
            Op {
                op: '+',
                spacing: Alone
            },
            Literal {
                lit: 1
            }
        ]
    }
]