rb_{thread,context,fiber}_struct: trivial packing

* vm_core.h (rb_thread_struct): reorder to pack
* cont.c (rb_context_struct, rb_fiber_struct): ditto
  On x86-64, these changes reduces:
  rb_thread_struct to 1000 bytes (from 1016)
  rb_context_struct to 1288 bytes (from 1312)
  rb_fiber_struct to 2272 bytes (from 2304)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47186 b2dd03c8-39d4-4d8f-98ff-823fe69b080e