The idea is to avoid allocating tuples when creating the fixpoint of
nary functions. For example, consider the new tests:
- tests/playground/fix.lean
- tests/playground/fix_with_tuples.lean
The second one (`fix_with_tuples`) uses the `fix` operator and tuples. For input = 20,
it creates more than 1 million extra objects. The first implementation
(`fix.lean`) using `fix₂` avoids this overhead.
TODO: Add support for pattern #N with N > 9 at
```
def expand_extern_pattern_aux (args : list string) : nat → string.iterator → string → string
```
