common: add generic input-line interface, and an implementation of it for text files

common/include/ivy/file.h

@@ -0,0 +1,15 @@
+#ifndef IVY_COMMON_FILE_H_
+#define IVY_COMMON_FILE_H_
+
+#include <ivy/line-source.h>
+#include <stdio.h>
+
+struct ivy_file {
+	struct ivy_line_source f_base;
+	FILE *f_fp;
+};
+
+extern struct ivy_file *ivy_file_from_fp(FILE *fp);
+extern void ivy_file_close(struct ivy_file *file);
+
+#endif