panama repo winter cleanup

Hi, as we are about to create a new branch (foreign-jextract) we took a look at the status of the various branches in panama/dev. We have decided to close the linkToNative-related branches: * linkToNative * foreign+linkToNative as these branches are no longer being worked on; all the new efforts in terms of enhancing foreign function VM support is happening in the foreign-abi branch, (see the ProgrammableInvoker effort [1]), which effectively brings together the best aspects of the legacy universal invoker (such as debuggability) and the linkToNative invoker (such as peak performances). Cheers Maurizio [1] - https://mail.openjdk.java.net/pipermail/panama-dev/2019-September/006347.html