split internal.h into files

One day, I could not resist the way it was written.  I finally started
to make the code clean.  This changeset is the beginning of a series of
housekeeping commits.  It is a simple refactoring; split internal.h into
files, so that we can divide and concur in the upcoming commits.  No
lines of codes are either added or removed, except the obvious file
headers/footers.  The generated binary is identical to the one before.

internal/inits.h

@@ -0,0 +1,51 @@
+#ifndef INTERNAL_INITS_H /* -*- C -*- */
+#define INTERNAL_INITS_H
+/**
+ * @file
+ * @brief      Internal header aggregating init functions.
+ * @author     \@shyouhei
+ * @copyright  This  file  is   a  part  of  the   programming  language  Ruby.
+ *             Permission  is hereby  granted,  to  either redistribute  and/or
+ *             modify this file, provided that  the conditions mentioned in the
+ *             file COPYING are met.  Consult the file for details.
+ */
+
+/* class.c */
+void Init_class_hierarchy(void);
+
+/* dmyext.c */
+void Init_enc(void);
+void Init_ext(void);
+
+/* file.c */
+void Init_File(void);
+
+/* gc.c */
+void Init_heap(void);
+
+/* localeinit.c */
+int Init_enc_set_filesystem_encoding(void);
+
+/* newline.c */
+void Init_newline(void);
+
+/* vm.c */
+void Init_BareVM(void);
+void Init_vm_objects(void);
+
+/* vm_backtrace.c */
+void Init_vm_backtrace(void);
+
+/* vm_eval.c */
+void Init_vm_eval(void);
+
+/* vm_insnhelper.c */
+void Init_vm_stack_canary(void);
+
+/* vm_method.c */
+void Init_eval_method(void);
+
+/* inits.c */
+void rb_call_inits(void);
+
+#endif /* INTERNAL_INITS_H */