Update perldelta for 5.17.8

Including the generated Acknowledgements section.

Thanks to Smylers++ for reviewing an earlier draft.
Last-minute "known issue" in perldelta
Update Module::CoreList for 5.17.8
Add 5.17.8 to perlhist
Add the 5.17.8 epigraph to epigraphs.pod
Make a new perldelta for 5.17.9-to-be
Bump the Perl version in various places for 5.17.9