simplifying the Panama/foreign repo

[Maurizio Cimadamore]

Hi,
As you might know, yesterday we integrated JEP 424 into jdk/jdk [1]. 
This is a great milestone, and I'd like to thank everybody (both inside 
and outside Oracle) that has helped along the way.

Moving forward, we would like to simplify the Panama/foreign repo [2] 
structure a bit. We currently have several active branch which are in 
need of some consolidation:

* foreign-memaccess+abi: this is where development of the memory access 
and linker API used to happen
* foreign-preview: this is a staging branch we've been using to 
accumulate the API preview changes (as well as some other important VM 
changes)
* foreign-jextract: this branch is the same as foreign-memaccess+abi, 
but with jextract added on top.

Given that most of the changes have been integrated into mainline 
(except for some JVM changes [3] that will be integrated soon), and 
given that jextract has now moved into its own standalone repo [4], I 
propose that, moving forward, we only keep the "foreign-memaccess+abi". 
This branch will be used, as before, to accumulate changes that will 
integrate in JDK N + 1 (e.g. for the next preview of the foreign API, or 
for when the foreign API will be finalized).

This should help minimizing confusion, as well as maintenance burden (as 
all the above branches need to be periodically synced against mainline).

Cheers
Maurizio

[1] - https://github.com/openjdk/jdk/pull/7888
[2] - https://github.com/openjdk/panama-foreign
[3] - https://git.openjdk.java.net/jdk/pull/7959
[4] - https://github.com/openjdk/jextract