determinism: Interface re-export list det

determinism: Interface re-export list det

In 'DocStructureItem' we want to make sure the 'Avails' are sorted, for
interface file determinism. This commit introduces 'SortedAvails', an
abstract newtype that can only be constructed by sorting Avails with
'sortAvails'. This newtype is used by 'DocStructureItem' where 'Avails'
was previously used to ensure the list of avails is deterministically
sorted by construction.

Note: Even though we order the constructors and avails in the interface
file, the order of constructors in the haddock output is still
determined from the order of declaration in the source. This was also
true before, when the list of constructors in the interface file <docs>
section was non-deterministic. Some haddock tests such as
"ConstructorArgs" observe this (check the order of constructors in
out/ConstructorArgs.html vs src/ConstructorArgs.hs vs its interface file)

The updated tests are caused by haddock corners where the order in the
source is not preserved (and was non-deterministic before this PR):
    * Module header in the latex backend
    * Re-export of pattern synonyms associated to a datatype (#25342)

Fixes #25304