Fix compilation errors for the newly introduce test & add type inference.

Tests are basically functions for which the return type should unify with bool. In principle, the type checker could also check that a test function has no arguments but, a test function with arguments wouldn't parse in the first place; feels a bit hacky but it works when considering the pipeline as a whole.

  Note that the code generation is still to be done.

examples/sample/validators/swap.ak

@@ -11,3 +11,7 @@ pub fn spend(datum: sample.Datum, rdmr: sample.Redeemer, _ctx: Nil) -> Bool {
 
   z == #(#[222], #[222])
 }
+
+test foo() {
+  1 + 1 == 2
+}