ArrowType.rigidify-sig! should detect cases where stack type base can be turned into STACK_TYPE_UNIT.

[typeswitch-dev]

Because STACK_TYPE_UNIT is treated specially when applying type signatures (during type checking), it would be better if rigidify-sig! detected cases where the stack base of an arrow type can be turned into STACK_TYPE_UNIT on both sides of the arrow type without loss of generality.

ArrowType.rigidify-sig! is only used currently in the specializer, where this isn't needed. But it's also useful for implementing type signature inference. Low priority.