[foreign-memaccess] RFR: Backport misc changes from foreign-abi

[Jorn Vernee]

Hi,

This patch backports several changes that went into the foreign-abi branch, but which are in retrospect useful to have
on the memaccess branch, and as part of the memory access API as well.

These changes include:
- Replacing ForeignUnsafe with Foreign
- Adding the `ofNativeUnchecked` method to create unchecked MemorySegments, MemoryAddress::ofLong to forge
  MemoryAddresses from longs, as well as the Nothing segment and rebase support. Mainly to support going from a `long` to
  `MemoryAddress`, and also to `MemorySegment` (MA -> long is already covered) The memory layout attributes exposed in
  the public API (could be interesting to investigate attaching carrier types directly to layouts).

The MemoryAddress carrier support for VarHandles is left out for now, since the current thinking is that it will be
dropped and replaced with VarHandle combinators to convert between long and MA instead.

Thanks,
Jorn

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Commit messages:
 - Backport misc changes from foreign-abi

Changes: https://git.openjdk.java.net/panama-foreign/pull/77/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/77/webrev.00
  Stats: 826 lines in 21 files changed: 655 ins; 96 del; 75 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/77.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/77/head:pull/77

PR: https://git.openjdk.java.net/panama-foreign/pull/77