[foreign-memaccess] RFR: Backport Foreign methods from foreign-abi back to foreign-memaccess

[Maurizio Cimadamore]

This is a straight backport for

https://git.openjdk.java.net/panama-foreign/pull/81

The only other thing I've fixed is a byte buffer test that was intermittently causing OOME. I realized that:

* the test was allocating an absurd amount of native memory (max int * 2)
* the test was not closing the native segment afterwards

Both issues are now addressed.

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Commit messages:
 - Backport Foreign methods from foreign-abi back to foreign-memaccess

Changes: https://git.openjdk.java.net/panama-foreign/pull/91/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/91/webrev.00
  Stats: 50 lines in 5 files changed: 27 ins; 7 del; 16 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/91.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/91/head:pull/91

PR: https://git.openjdk.java.net/panama-foreign/pull/91