Upgrade List.sort builtins

[lukewilliamboswell]

Zulip discussion

Supersedes #6988
Supersedes #6989

sort: List elem -> List elem where elem implements Sort
sortBy: List elem, (elem -> field) -> List elem where field implements Sort
sortWith: List elem, (elem, elem -> [ LessThan, Equal, GreaterThan ]) -> List elem

# Then the reversed versions:
sortReversed: List elem -> List elem where elem implements Sort
sortByReversed: List elem, (elem -> field) -> List elem where field implements Sort
sortWithReversed: List elem, (elem -> [ LessThan, Equal, GreaterThan ]) -> List elem

Blocked until #6615 is implemented.