Fix missing MRB_API prefix for functions below; clse #4267

Functions to add prototypes to headers:
* mrb_ary_splice()
* mrb_notimplement()
* mrb_vformat()
* mrb_cstr_to_dbl()
* mrb_cstr_to_inum()

Functions to be made `static` (`MRB_API` was not needed):
* mrb_mod_module_function()
* mrb_obj_hash()
* mrb_str_len_to_inum()

Functions to remove `MRB_API` from definitions (referenced from within `libmruby`):
* mrb_mod_cv_defined()
* mrb_mod_cv_get()
* mrb_f_send()

include/mruby/array.h

@@ -227,6 +227,22 @@ MRB_API mrb_value mrb_ary_unshift(mrb_state *mrb, mrb_value self, mrb_value item
  */
 MRB_API mrb_value mrb_ary_entry(mrb_value ary, mrb_int offset);
 
+/*
+ * Replace subsequence of an array.
+ *
+ * Equivalent to:
+ *
+ *      ary.shift
+ *
+ * @param mrb The mruby state reference.
+ * @param self The array from which the value will be shifted.
+ * @param head Beginning position of a replacement subsequence.
+ * @param len Length of a replacement subsequence.
+ * @param rpl The array of replacement elements.
+ * @return The receiver array.
+ */
+MRB_API mrb_value mrb_ary_splice(mrb_state *mrb, mrb_value self, mrb_int head, mrb_int len, mrb_value rpl);
+
 /*
  * Shifts the first element from the array.
  *