isthmus: Lambda support limitation

[limameml]

As of now Calcite doesn't support nested lambda expressions like these:

# Represents: `(outer_x: i32) -> ((inner_y: i32) -> add(outer_x, inner_y))`
# Demonstrates steps_out:
#   - steps_out: 1 with struct_field: 0 -> outer_x
#   - steps_out: 0 with struct_field: 0 -> inner_y

So for the moment in the isthmus implementation, we error on nested lambda use cases (steps_out>0), this ticket will pick up the implementation once the calcite support is available.