Simplify binop parsing

[irevoire]

Hey, just a little PR that tries to simplify adding new binop operations.

It's not as worth it as I thought though, so feel free to close it if you think it actually makes the code harder to read.

Having two closures in the parse_binop method is a bit hard to use IMO, but overall, being able to use the same method for 7 other parsers is interesting.

[sharkdp]

I had the same thought when I reviewed your logical-operator PR — nice! If there are no performance regressions, I'm happy to accept this. I think it's definitely an improvement.

[irevoire]

On my side, I didn't measure any performance regression:

Import prelude          time:   [16.242 ms 16.293 ms 16.345 ms]
                        change: [-0.3828% +0.1170% +0.5889%] (p = 0.63 > 0.05)
                        No change in performance detected.