[Request]Precipitation and gas evolution arrows

[Ants-Aare]

I'm not sure yet what a good shorthand way of writing this would be. we are already using the ^ symbol for adding charges to elements, could become confusing if we used it for precipitation. mhchem uses (^) for gas and v for precipitation. I kind of like the v but dislike the bracketed ^

[tryptophawa]

Maybe just use a rule that switches between:
(g) -> ↑ ; (s) -> ↓ and (g) -> (g) ; (s) -> (s)