constant.h: constant visibilities

* constant.h (RB_CONST_PRIVATE_P, RB_CONST_PUBLIC_P): macros to
  predicate constant visibilities.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47846 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
variable.c: use st_update

* variable.c (sv_i, cv_i): use st_update to insert non-existing
  entries.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47847 b2dd03c8-39d4-4d8f-98ff-823fe69b080e