compiler: Flesh Out CST Visitor Framework

As a use case, use it to implement text serialization
of the CST.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Tests/p4rseTests/CodeGenTests/Serializer.swift

@@ -46,5 +46,30 @@ import TreeSitterP4
   let v = CSTTextSerializer()
   let c = CSTTextSerializerContext();
   let vd = CSTVisitorDriver();
-  #expect(#RequireOkResult((vd.visit(program: program, visitor: v, context: c))))
+  let result = try #UseOkResult((vd.start(program: program, visitor: v, context: c)))
+
+  let expected = """
+	Parser Expression
+		State: Direct Transition
+			Statements:
+				Expression Statement:
+					Literal Expression
+			Next State:
+				Identifier: accept
+		State: Select Transition
+			Select Expression:
+				Selector:
+					Literal Expression
+				Case Expressions:
+					Case Expression:
+						Keyset Expression:
+						Next State:
+							Identifier: accept
+					Case Expression:
+						Keyset Expression:
+						Next State:
+							Identifier: reject
+
+	"""
+  #expect(result.serialized == expected)
 }