feat: Allow empty (begin ..) (#289)

This adjusts the minimum expected arity for `begin` statements to allow
them to be empty. The reason for this is that, in practice, we may have
a `begin` statement stuffed only with `debug` constraints (which are
physically removed unless the `--debug` switch is provided).

src/compiler/generator.rs

@@ -251,7 +251,7 @@ impl FuncVerifier<Node> for Intrinsic {
             Intrinsic::Neg => Arity::Monadic,
             Intrinsic::Inv => Arity::Monadic,
             Intrinsic::Normalize => Arity::Monadic,
-            Intrinsic::Begin => Arity::AtLeast(1),
+            Intrinsic::Begin => Arity::AtLeast(0),
             Intrinsic::IfZero | Intrinsic::IfNotZero => Arity::Between(2, 3),
         }
     }