evaluate sum on binary list

[jvoigtlaender]

Seen in student code.

(Of course, similar evaluation rules like sum [x] -> x could also be added. But this is the one I have really seen.)

[jvoigtlaender]

The new hint is also in the spirit of existing ones like these:

hlint/data/hlint.yaml

Lines 132 to 133 in 752baba

	- warn: {lhs: "maximum [a, b]", rhs: max a b}
	- warn: {lhs: "minimum [a, b]", rhs: min a b}

[ndmitchell]

Thanks!