[foreign-memaccess+abi] RFR: Simplify ResourceScope::acquire for implicit scopes

[Maurizio Cimadamore]

As suggested in [1], we can make the API for acquiring implicit scopes simpler if we tweak the implementation so that implicit scopes also implement the `HandleImpl` interface. If we do that, we could implement `acquire` by simply returning the implicit scope instance.

The simplification is that now, from a client perspective, acquiring a scope always gives back an handle that is associated with the original scope - we no longer *cheat* using the implicit scope handle singleton.

The acquire performance are not affected by this change (I've ran `BulkMismatchAcquire` before and after).

[1] - https://mail.openjdk.java.net/pipermail/panama-dev/2021-April/013516.html

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Commit messages:
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/523/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=523&range=00
  Stats: 42 lines in 5 files changed: 6 ins; 18 del; 18 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/523.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/523/head:pull/523

PR: https://git.openjdk.java.net/panama-foreign/pull/523