vm_core.h (struct rb_iseq_struct): stack_max is uint32_t

* vm_core.h (struct rb_iseq_struct): stack_max is uint32_t
  No program will ever need more than a few megabytes of stack,
  so there's no sense in using a 64-bit counter for stack
  accounting.  Packing this with the 32-bit type enum reduces
  rb_iseq_struct from 312 to 304 bytes on 64-bit systems.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46798 b2dd03c8-39d4-4d8f-98ff-823fe69b080e