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/eval.h

@@ -0,0 +1,30 @@
+#ifndef INTERNAL_EVAL_H /* -*- C -*- */
+#define INTERNAL_EVAL_H
+/**
+ * @file
+ * @brief      Internal header for the evaluator.
+ * @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.
+ * @note       There  also  is  eval_intern.h, which  is  evaluator's  internal
+ *             header (related to this file, but not the same role).
+ */
+
+/* eval.c */
+VALUE rb_refinement_module_get_refined_class(VALUE module);
+extern ID ruby_static_id_signo, ruby_static_id_status;
+void rb_class_modify_check(VALUE);
+#define id_signo ruby_static_id_signo
+#define id_status ruby_static_id_status
+NORETURN(VALUE rb_f_raise(int argc, VALUE *argv));
+
+/* eval_error.c */
+VALUE rb_get_backtrace(VALUE info);
+
+/* eval_jump.c */
+void rb_call_end_proc(VALUE data);
+void rb_mark_end_proc(void);
+
+#endif /* INTERNAL_EVAL_H */