panama repo winter cleanup

[Maurizio Cimadamore]

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