range.c: setter macros

* range.c (RANGE_SET_{BEG,END,EXCL}): add setter macros which wrap
  RSTRUCT_SET() and index.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43419 b2dd03c8-39d4-4d8f-98ff-823fe69b080e