Function used as a type but is in fact a term

[NeuralCoder3]

Observed in lit/direct/ds2cps_ax_cps2ds_dependent_short.thorin
Command line: ./build/bin/thorin -d direct -d tool ./lit/direct/ds2cps_ax_cps2ds_dependent_short.thorin --output-thorin -

Issue:

:4294967295: error: 'Π n_167425: .Nat → .infer (<nullptr>)' used as a type but is in fact a term 

High-level description:
We constructed a wrapper around .Idx

Suspected problem:
Idx has problems with the duality of fundamental integer and being applied as a function.

[leissa]

Suffers also from #106.

• changing the literal to 0:.Idx n and
• adding a return type to lam_idx fixes the test case.

see also #103:

Work is underway to add type inference.

[leissa]

Marked as enhancement as the only remaining real problem is the type inference that is not yet properly implemented.