struct: avoid all O(n) behavior on access

This avoids O(n) on lookups with structs over 10 members.
This also avoids O(n) behavior on all assignments on Struct members.
Members 0..9 still use existing C methods to read in O(1) time

Benchmark results:

vm2_struct_big_aref_hi*	1.305
vm2_struct_big_aref_lo*	1.157
vm2_struct_big_aset*	3.306
vm2_struct_small_aref*	1.015
vm2_struct_small_aset*	3.273

Note: I chose use loading instructions from an array instead of writing
directly to linked-lists in compile.c for ease-of-maintainability.  We
may move the method definitions to prelude.rb-like files in the future.

I have also tested this patch with the following patch to disable
the C ref_func methods and ensured the test suite and rubyspec works

	--- a/struct.c
	+++ b/struct.c
	@@ -209,7 +209,7 @@ setup_struct(VALUE nstr, VALUE members)
		ID id = SYM2ID(ptr_members[i]);
		VALUE off = LONG2NUM(i);

	-	if (i < N_REF_FUNC) {
	+	if (0 && i < N_REF_FUNC) {
		    rb_define_method_id(nstr, id, ref_func[i], 0);
		}
		else {

* iseq.c (rb_method_for_self_aref, rb_method_for_self_aset):
  new methods to generate bytecode for struct.c
  [Feature #10575]
* struct.c (rb_struct_ref, rb_struct_set): remove
  (define_aref_method, define_aset_method): new functions
  (setup_struct): use new functions
* test/ruby/test_struct.rb: add test for struct >10 members
* benchmark/bm_vm2_struct_big_aref_hi.rb: new benchmark
* benchmark/bm_vm2_struct_big_aref_lo.rb: ditto
* benchmark/bm_vm2_struct_big_aset.rb: ditto
* benchmark/bm_vm2_struct_small_aref.rb: ditto
* benchmark/bm_vm2_struct_small_aset.rb: ditto

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48748 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_struct.rb

@@ -186,6 +186,15 @@ module TestStruct
     assert_raise(ArgumentError) { o.select(1) }
   end
 
+  def test_big_struct
+    klass1 = @Struct.new(*('a'..'z').map(&:to_sym))
+    o = klass1.new
+    assert_nil o.z
+    assert_equal(:foo, o.z = :foo)
+    assert_equal(:foo, o.z)
+    assert_equal(:foo, o[25])
+  end
+
   def test_equal
     klass1 = @Struct.new(:a)
     klass2 = @Struct.new(:a, :b)