bump version to 5.20.0, install 5.20 perldelta
perldelta: correct busted perldiag links
regenerate META.json
add known modules from perl520delta
resort win32/pod.mak to please pod_rules.t
replace 5.19.12 (never happening) with 5.20.0 in corelist
there are no longer any problems in perldelta for now
patchlevel: call it RC0 until RC1
bump the version to 5.20.0-RC0 to start on the road to release!