Opimtize 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}
```
1 file changed