Optimize key tree stemming by using maps instead of sets

sets to due to a compiler bug in 22 consumes too much memory and is
slower on Erlang 22+

Reproducer: https://gist.github.com/nickva/ddc499e6e05678faf20d344c6e11daaa

With sets:
```
couch_key_tree:gen_and_stem().
{8,6848.812683105469}
```

With maps:
```
couch_key_tree:gen_and_stem().
{0,544.000732421875}
```

This is a cherry-pick of https://github.com/apache/couchdb/commit/91de482fd66f4773b3b8583039c6bcaf1c5727ec

1 files changed, 5 insertions, 5 deletions

diff --git a/src/couch/src/couch_key_tree.erl b/src/couch/src/couch_key_tree.erl
index 94150418e..7811d9532 100644
--- a/src/couch/src/couch_key_tree.erl
+++ b/src/couch/src/couch_key_tree.erl
@@ -471,7 +471,7 @@ stem(Trees, Limit) ->
         {_, Branches} = lists:foldl(fun(Tree, {Seen, TreeAcc}) ->
             {NewSeen, NewBranches} = stem_tree(Tree, Limit, Seen),
             {NewSeen, NewBranches ++ TreeAcc}
-        end, {sets:new(), []}, Trees),
+        end, {#{}, []}, Trees),
         lists:sort(Branches)
     catch throw:dupe_keys ->
         repair_tree(Trees, Limit)
@@ -522,11 +522,11 @@ stem_tree(Depth, {Key, Val, Children}, Limit, Seen0) ->
 
 
 check_key(Key, Seen) ->
-    case sets:is_element(Key, Seen) of
-        true ->
+    case Seen of
+        #{Key := true} ->
             throw(dupe_keys);
-        false ->
-            sets:add_element(Key, Seen)
+        _ ->
+            Seen#{Key => true}
     end.