* string.c (rb_str_succ): add missing case NEIGHBOR_WRAPPED.
  r42078 caused buggy behavior like "\xFF".b -> "\x01\xFF".b

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42082 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
rename a() to b() and define a() for US-ASCII

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42083 b2dd03c8-39d4-4d8f-98ff-823fe69b080e