[foreign-memaccess] RFR: Add benchmark to measure performance of VH adapters

[Maurizio Cimadamore]

This patch adds a straightforward benchmark to measure performances of var handle adapters.

The benchmark is set up to read values from an int array, both with a var handle(obtained from `MethodHandles`), an
associated `MethodHandle` (derived from the first var handle) and with a segment-based, memory access var handle.

Then the same test is repeated, but with an extra adaptation step inserted in the middle - rather than reading `int`
values directly, the adapter turns `ints` into instances of `IntBox` and the loop code doing the sum converts them back
into ints.

Numbers are extremeluy solid on my machine:

TestAdaptVarHandles.mh_box_loop       avgt   30  0.306 ? 0.009  ms/op
TestAdaptVarHandles.mh_loop           avgt   30  0.297 ? 0.009  ms/op
TestAdaptVarHandles.segment_box_loop  avgt   30  0.308 ? 0.009  ms/op
TestAdaptVarHandles.segment_loop      avgt   30  0.307 ? 0.008  ms/op
TestAdaptVarHandles.vh_box_loop       avgt   30  0.296 ? 0.005  ms/op
TestAdaptVarHandles.vh_loop           avgt   30  0.291 ? 0.003  ms/op

I thought it would have been nice to add this to our benchmark suites since we do not have anything that tests VH
adaptation directly.

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Commit messages:
 - Add benchmark to measure performance of VH adapters

Changes: https://git.openjdk.java.net/panama-foreign/pull/175/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/175/webrev.00
  Stats: 161 lines in 1 file changed: 161 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/175.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/175/head:pull/175

PR: https://git.openjdk.java.net/panama-foreign/pull/175