refactor: Split ast into ast/syntax and ast/print

lib/ast/syntax.ml

@@ -0,0 +1,12 @@
+(* lib/ast/syntax.ml *)
+
+type binary_operator = Add | Substract | Multiply | Divide
+type unary_operator = Negate
+
+type expression =
+  | IntegerLiteral of int
+  | UnaryExpression of unary_operator * expression
+  | BinaryExpression of binary_operator * expression * expression
+
+type statement = ExpressionStatement of expression
+type source_file = SourceFile of statement list