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
```
Motivation: to make progress in the new compiler stack, we have
to preserve join points during lambda lifting. Right now, they are
lifted as regular lambdas. So, to keep them, we need some basic
support for them in the old VM. The implementation here is quick and
dirty. This is not an issue since this code will be deleted soon.
This commit also add two new instructions to the old VM: `updt` and
`updt_cidx`.
They allow us to test the new new memory reuse technique.
TODO: We need to reset fields after we project. Otherwise, we prevent
destructive updates on nested objects.
