fix: struct conversion

Author: kelvin-olaiya

src/main/scala/io/github/kelvindev15/prolog/core/Struct.scala

@@ -24,7 +24,7 @@ object Struct:
 
   private case class StructImpl(functor: Atom, arguments: Term*) extends Struct:
     override val arity: Int = arguments.size
-    override def asTerm: Term = this
+    override def asTerm: Term = Struct(functor, arguments map { _.asTerm }*)
 
   trait Indicator extends Struct:
     val functor: Atom = Functors.INDICATOR

src/test/scala/io/github/kelvindev15/prolog/core/TestTermConversion.scala

@@ -21,7 +21,7 @@ class TestTermConversion extends AnyFunSuite with Matchers:
 
   test("A directive is a unary struct with :- as a functor"):
     Directive(Struct(Atom("dynamic"), Indicator(Atom("foo"), Constant.Numeric(2)))).asTerm shouldBe
-      Struct(Atom(":-"), Struct(Atom("/"), Atom("foo"), Constant.Numeric(2)))
+      Struct(Atom(":-"), Struct(Atom("dynamic"), Struct(Atom("/"), Atom("foo"), Constant.Numeric(2))))
 
   test("A conjunction of goals is a binary recursive struct with , as a functor"):
     val functor = Atom(",")