prefer Sub::Util to Sub::Name

prefer Sub::Util to Sub::Name

Sub::Util can name subs as well, and is in core in newer versions.  Try
to use it instead of Sub::Name, although prefer Sub::Name if it is
already loaded.

Each module has some edge cases though.  Sub::Name has a memory leak
prior to 0.08.  Some systems can end up with a version of Sub::Util that
doesn't have the set_subname sub.  Check for these cases as well when
picking a module to use.

Also optimize out the sub naming if neither module is available.