Implement listSorted

[konnov]

From time to time, we need an operator that sorts a list. I started to write one some time ago, but than ran into a few unsupported features. It looks like we have to implement #150 first.

This work is tracked in: https://github.com/informalsystems/quint/blob/igor/sorting/examples/spells/basicSpells.qnt#L118

This definition was asked for by @andrey-kuprianov and @angbrav.

[bugarela]

I don't understand this issue. It already refers to the implementation, so seems like it is completed already.

[shonfeder]

I think that is a link to a WIP branch.

[bugarela]

ah right! I didn't notice that, sorry!