Print source snippet for lookahead value of rr-conflicts

[ratmice]

This adds printing using spans for the lookahead information that was added in #506
using the example from that PR

%start S
%%
S: A X | B X | A Y;
A: '0' ; B: '0' ; C: '0' ;
X: '1' ; Y: '2' ;

Adds the last line to this conflict printing output:

Reduce/Reduce conflict, can reduce 'A' or 'B' with lookahead '1' at rr.y:5:1
5| A: '0' ; B: '0' ; C: '0' ;
   ^ First reduce
5| A: '0' ; B: '0' ; C: '0' ;
            - Second reduce
6| X: '1' ; Y: '2' ;
       + Lookahead

Not certain we need to be using different underline characters for each thing here, but it was currently doing that so I kept the tradition?

[ltratt]

Not certain we need to be using different underline characters for each thing here, but it was currently doing that so I kept the tradition?

Hmm, probably it would be better to use ^ for all of them (which seems more consistent with other tools)? But we can do that in a follow-up PR if you agree.

[ratmice]

Yeah, I think my original attempts probably had more of a footnote style attempting to reduce duplicate line output.
By having multiple underlines on a single line, and then footnote text on their own line.

But as we can see from the duplicate line output that isn't actually being done, so I agree that keeping to ^ is probably better.

[ratmice]

Force pushed a clippy/rustfmt update