toolchain: add a program for compiling ipc interface definitions

ifc can be used to compile .if files into self-contained header-only C
libraries, which can be used to send/receive messages that conform
to the described interface.

toolchain/ifc/file-span.h

@@ -0,0 +1,12 @@
+#ifndef IFC_FILE_SPAN_H_
+#define IFC_FILE_SPAN_H_
+
+struct file_cell {
+	unsigned int c_row, c_col;
+};
+
+struct file_span {
+	struct file_cell s_start, s_end;
+};
+
+#endif