[foreign-abi] RFR: 8241017: Enhance AllocationScope to support "unbounded" mode

[Maurizio Cimadamore]

This patch adds support for allocation scopes whose size is not known statically; as such these new unbounded
allocation scope can be used in more dynamic use cases where e.g. the amount of memory to be allocated depends on the
result of other native calls.

Internally, the bounded version works as before, and is the more optimized. The unbounded version is similar in spirit
to the old Panama/foreign scope - where new segments are allocated depending on needs.

I've enhanced the existing test covering allocation scopes to also cover the unbounded variants, as well as the bounded.

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Commit messages:
 - JDK-8241017: Enhance AllocationScope to support "unbounded" mode

Changes: https://git.openjdk.java.net/panama-foreign/pull/53/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/53/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8241017
  Stats: 320 lines in 4 files changed: 240 ins; 25 del; 55 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/53.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/53/head:pull/53

PR: https://git.openjdk.java.net/panama-foreign/pull/53