[foreign-memaccess] RFR: Move MemoryAddress::copy

[Maurizio Cimadamore]

As discussed, this patch moves `MemoryAddress::copy` as an *instance* method of `MemorySegment`, namely
`MemorySegment::copyFrom`. I chose the `copyFrom` suggestion from John, since it allows us more freedom, in the future,
to add overloads for different sources for the copy operation.

I've rewrorked the documentation a bit to speak about segment offsets rather than addresses.

I believe all use cases touched by this change actually lead to simpler code, which is good.

I've also added a benchmark for some of the bulk operations such as fill and copyFrom - the numbers I've got look good:

Benchmark             Mode  Cnt       Score       Error  Units
BulkOps.segment_copy  avgt   30  441675.750 ? 12794.881  ns/op
BulkOps.segment_fill  avgt   30  122465.908 ?  1760.711  ns/op
BulkOps.unsafe_copy   avgt   30  430153.694 ?  9055.253  ns/op
BulkOps.unsafe_fill   avgt   30  120413.079 ?  3799.484  ns/op

Cheers

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Commit messages:
 - Added benchmark for bulk operations
 - Move static MemoryAddress::copy into instance method inside MemorySegment

Changes: https://git.openjdk.java.net/panama-foreign/pull/169/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/169/webrev.00
  Stats: 172 lines in 8 files changed: 130 ins; 38 del; 4 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/169.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/169/head:pull/169

PR: https://git.openjdk.java.net/panama-foreign/pull/169