* include/ruby/ruby.h: support write barrier protection for T_STRUCT.
  Introduce the following C APIs:
  * RSTRUCT_RAWPTR(st) returns pointer (do WB on your risk).
    The type of returned pointer is (const VALUE *).
  * RSTRUCT_GET(st, idx) returns idx-th value of struct.
  * RSTRUCT_SET(st, idx, v) set idx-th value by v with WB.
  And
  * RSTRUCT_PTR(st) returns pointer with shady operation.
    The type of returned pointer is (VALUE *).
* struct.c, re.c, gc.c, marshal.c: rewrite with above APIs.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41534 b2dd03c8-39d4-4d8f-98ff-823fe69b080e