[foreign-memaccess+abi] RFR: Investigate alternate strategy to acquire resource scopes [v3]

[Maurizio Cimadamore]

> The code in this PR demonstrates a possible, alternate approach to resource scope acquire.
> 
> Instead of having `ResourceScope.Handle` be `AutoCloseable` (which is problematic, given that `javac` issues warning when an auto closeable is not used inside the body of the try block) this patch adds a new method on resource scopes, namely `ResourceScope::release(Handle)`.
> 
> The semantics of this method is similar to the previous `Handle::close` - one thing that is different is how implicit scopes are handled - instead of trying to keep the implicit scope alive through the handle instance, here we simply return a "dummy" handle, which does nothing.
> 
> The idea is that if you write idiomatic code like:
> 
> 
> var handle = scope.acquire();
> try {
>    // critical region
> } finally {
>    scope.release(handle); 
> } 
> 
> 
> This will work for both explicit scopes (since acquiring actively prevents calls to `ResourceScope::close`) - but also for implicit scopes, since the `release` call inside the finally block will effectively require the scope to be reachable at this stage (see reachability fence inside `ResourceScopeImpl::release`).

Maurizio Cimadamore has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains three commits:

 - Merge branch 'foreign-memaccess+abi' into acquire_release_scope
 - Add reachability fence on ResourceScope::release
 - Initial push

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Changes: https://git.openjdk.java.net/panama-foreign/pull/511/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=511&range=02
  Stats: 120 lines in 9 files changed: 74 ins; 8 del; 38 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/511.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/511/head:pull/511

PR: https://git.openjdk.java.net/panama-foreign/pull/511