[foreign-memaccess+abi] RFR: Improve performance of Arena::allocateFrom

[Maurizio Cimadamore]

This patch improves the performance of allocation of a standard confined/shared arenas in two steps:

* first, it special cases the allocation methods in SegmentAllocator to detect the case where the SegmentAllocator implementation is the internal Arena implementation. In such case, all the `allocateFrom` methods attempt an allocation request which does not perform memory zeroing (as the contents are going to be overwritten anyway).
* second, it minimizes the overhead associated with reserving/unreserving memory. More specifically, it only calls Bits::reserveMemory/unreserveMemory when allocating from an automatic arena.

Implementation-wise, this is done by having an internal arena implementation class (`ArenaImpl`) which is the implementation returned by the various arena factories. This class will have methods to allocate zeroed memory and non-zeroed memory. In order to avoid duplication of the various allocation routines, I instead re-routed the SegmentAllocator methods to a private allocation implementation which sees if we're `ArenaImpl` and if so calls the implementation that has better knowledge. Alternatively I could have overridden all `allocateFrom` methods from `SegmentAllocator` but that would have required some duplication.

One caveat: a custom arena does NOT inherit this special behavior. That is, it is the responsibility of the custom arena to define "shortcut" for the `allocateFrom` methods. The only way to avoid that would be to have zeroing as an explicit boolean parameter in the allocation methods, but that's not very safe, as it is now up to client to decide if they want zeroing or not. That said, this is only an issue for custom arenas, and we can assume that a client that wants a specialized arena behavior can handle overriding a bunch of methods via delegation (in case they care).

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Commit messages:
 - Add copyright
 - Revert spurious changes in test
 - Remove unused method in SegmentAllocator
 - Remove redundant imports in SegmentAllocator
 - Remove changes to Bits
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/855/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=855&range=00
  Stats: 339 lines in 7 files changed: 289 ins; 22 del; 28 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/855.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/855/head:pull/855

PR: https://git.openjdk.org/panama-foreign/pull/855