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/array.h

@@ -0,0 +1,88 @@
+#ifndef INTERNAL_ARRAY_H /* -*- C -*- */
+#define INTERNAL_ARRAY_H
+/**
+ * @file
+ * @brief      Internal header for Array.
+ * @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.
+ */
+
+/* array.c */
+
+#ifndef ARRAY_DEBUG
+#define ARRAY_DEBUG (0+RUBY_DEBUG)
+#endif
+
+#ifdef ARRAY_DEBUG
+#define RARRAY_PTR_IN_USE_FLAG FL_USER14
+#define ARY_PTR_USING_P(ary) FL_TEST_RAW((ary), RARRAY_PTR_IN_USE_FLAG)
+#else
+
+/* disable debug function */
+#undef  RARRAY_PTR_USE_START_TRANSIENT
+#undef  RARRAY_PTR_USE_END_TRANSIENT
+#define RARRAY_PTR_USE_START_TRANSIENT(a) ((VALUE *)RARRAY_CONST_PTR_TRANSIENT(a))
+#define RARRAY_PTR_USE_END_TRANSIENT(a)
+#define ARY_PTR_USING_P(ary) 0
+
+#endif
+
+#if USE_TRANSIENT_HEAP
+#define RARY_TRANSIENT_SET(ary) FL_SET_RAW((ary), RARRAY_TRANSIENT_FLAG);
+#define RARY_TRANSIENT_UNSET(ary) FL_UNSET_RAW((ary), RARRAY_TRANSIENT_FLAG);
+#else
+#undef RARRAY_TRANSIENT_P
+#define RARRAY_TRANSIENT_P(a) 0
+#define RARY_TRANSIENT_SET(ary) ((void)0)
+#define RARY_TRANSIENT_UNSET(ary) ((void)0)
+#endif
+
+VALUE rb_ary_last(int, const VALUE *, VALUE);
+void rb_ary_set_len(VALUE, long);
+void rb_ary_delete_same(VALUE, VALUE);
+VALUE rb_ary_tmp_new_fill(long capa);
+VALUE rb_ary_at(VALUE, VALUE);
+VALUE rb_ary_aref1(VALUE ary, VALUE i);
+size_t rb_ary_memsize(VALUE);
+VALUE rb_to_array_type(VALUE obj);
+VALUE rb_check_to_array(VALUE ary);
+VALUE rb_ary_tmp_new_from_values(VALUE, long, const VALUE *);
+VALUE rb_ary_behead(VALUE, long);
+#if defined(__GNUC__) && defined(HAVE_VA_ARGS_MACRO)
+#define rb_ary_new_from_args(n, ...) \
+    __extension__ ({ \
+        const VALUE args_to_new_ary[] = {__VA_ARGS__}; \
+        if (__builtin_constant_p(n)) { \
+            STATIC_ASSERT(rb_ary_new_from_args, numberof(args_to_new_ary) == (n)); \
+        } \
+        rb_ary_new_from_values(numberof(args_to_new_ary), args_to_new_ary); \
+    })
+#endif
+
+static inline VALUE
+rb_ary_entry_internal(VALUE ary, long offset)
+{
+    long len = RARRAY_LEN(ary);
+    const VALUE *ptr = RARRAY_CONST_PTR_TRANSIENT(ary);
+    if (len == 0) return Qnil;
+    if (offset < 0) {
+        offset += len;
+        if (offset < 0) return Qnil;
+    }
+    else if (len <= offset) {
+        return Qnil;
+    }
+    return ptr[offset];
+}
+
+RUBY_SYMBOL_EXPORT_BEGIN
+/* array.c (export) */
+void rb_ary_detransient(VALUE a);
+VALUE *rb_ary_ptr_use_start(VALUE ary);
+void rb_ary_ptr_use_end(VALUE ary);
+RUBY_SYMBOL_EXPORT_END
+
+#endif /* INTERNAL_ARRAY_H */