hash / hash-ext: various small changes

src/hash.c:
 - mrb_hash_(aget|aset|dup|delete): internal linkage
 - remove documentation of methods which are not implemented (in here)
 - remove #assoc; unused, not in ISO spec
 - remove #rassoc: same, implementation is also wrong

hash-ext mrbgem:
 - remove header "mruby/khash.h"
 - remove mrb_hash_values_at (move code into hash_values_at, i: long -> int)
 - less whitespace in gem_init function

mrbgems/mruby-hash-ext/src/hash-ext.c

@@ -7,7 +7,6 @@
 #include "mruby.h"
 #include "mruby/array.h"
 #include "mruby/hash.h"
-#include "mruby/khash.h"
 
 /*
  * call-seq:
@@ -20,27 +19,18 @@
  *   h.values_at("cow", "cat")  #=> ["bovine", "feline"]
  */
 
-mrb_value
-mrb_hash_values_at(mrb_state *mrb, int argc, mrb_value *argv, mrb_value hash)
-{
-    mrb_value result = mrb_ary_new_capa(mrb, argc);
-    long i;
-
-    for (i=0; i<argc; i++) {
-        mrb_ary_push(mrb, result, mrb_hash_get(mrb, hash, argv[i]));
-    }
-    return result;
-}
-
 static mrb_value
 hash_values_at(mrb_state *mrb, mrb_value hash)
 {
-  mrb_value *argv;
-  int argc;
+  mrb_value *argv, result;
+  int argc, i;
 
   mrb_get_args(mrb, "*", &argv, &argc);
-
-  return mrb_hash_values_at(mrb, argc, argv, hash);
+  result = mrb_ary_new_capa(mrb, argc);
+  for (i = 0; i < argc; i++) {
+    mrb_ary_push(mrb, result, mrb_hash_get(mrb, hash, argv[i]));
+  }
+  return result;
 }
 
 void
@@ -49,7 +39,6 @@ mrb_mruby_hash_ext_gem_init(mrb_state *mrb)
   struct RClass *h;
 
   h = mrb->hash_class;
-
   mrb_define_method(mrb, h, "values_at", hash_values_at, MRB_ARGS_ANY());
 }