* include/ruby/ruby.h: add new utility macros to access
  Array's element.
* RARRAY_AREF(a, i) returns i-th element of an array `a'
* RARRAY_ASET(a, i, v) set i-th element of `a' to `v'
  This change is a part of RGENGC branch [ruby-trunk - Feature #8339].



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40689 b2dd03c8-39d4-4d8f-98ff-823fe69b080e