started implementing expressions

Parser.parse now return list of statements or list of errors.
parsing continues until EOF, even when errors are found; but after the
first error the result can only be Error.
Also implemented Print and Expr statements.

lib/stmt.ml

@@ -0,0 +1,10 @@
+type stmt = Expr of Expr.expr_node | Print of Expr.expr_node
+and stmt_node = { stmt : stmt; pos : Error.code_pos }
+
+let make_expr_stmt (pos : Error.code_pos) (expr : Expr.expr_node) : stmt_node =
+  let stmt = Expr expr in
+  { stmt; pos }
+
+let make_print (pos : Error.code_pos) (expr : Expr.expr_node) : stmt_node =
+  let stmt = Print expr in
+  { stmt; pos }