strftime. It has some limitations that is same as Time's
one. [experimental]
* ext/date/date_strftime.c: new.
* ext/date/lib/date/format.c: removed ruby version of strftime.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@31135 b2dd03c8-39d4-4d8f-98ff-823fe69b080e