[foreign] repo update

[Maurizio Cimadamore]

Hi,
following integration of JEP 393 and 389, it doesn't really make sense 
to keep memory access and foreign linker on two separate branches - in 
fact, doing so just makes merges more awkward going forward. For this 
reason, we have decided to create a _new_ branch: foreign-memaccess+abi. 
The new branch contains both the foreign memory access API _and_ the 
foreign linker API and is the new "parent" branch for the 
foreign-jextract branch. From now on, new development on foreign memory 
access/foreign linker API will happen in this new unified branch. The 
old branches (foreign-memaccess and foreign-abi) will be left in place 
for the time being, so as to preserve history. We made sure to update 
references to the repo layout in both the Panama openjdk page [1] and in 
the GitHub Panama README file [2].

Cheers
Maurizio

[1] - https://openjdk.java.net/projects/panama/
[2] - https://github.com/openjdk/panama-foreign