[foreign-memaccess+abi] RFR: 8274978: Add implicit allocator factory

[Maurizio Cimadamore]

When rewriting tests, and also jextract, we found that the "implicit" segment allocator - that is, an allocator which returns native segments backed by fresh implicit scopes, is used quite frequently.

This patch adds a blessed factory for it, and it beefs up the allocator test, so that all allocators are tested (except for prefix, which is kind of peculiar).

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Commit messages:
 - Remove whitespaces
 - Add implicitAllocator

Changes: https://git.openjdk.java.net/panama-foreign/pull/595/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=595&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274978
  Stats: 219 lines in 2 files changed: 26 ins; 92 del; 101 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/595.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/595/head:pull/595

PR: https://git.openjdk.java.net/panama-foreign/pull/595