RFR: make FieldLayout#data() public

[Ivan Ponomarev]

Can we have this property public? This is needed for https://github.com/atp-mipt/ljv visualizer: we can use JOL to get object fields in consistent order, but then we also need `FieldData` itself in order to get fields' values

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Commit messages:
 - add specific accessors for field and field value
 - make FieldLayout#data() public

Changes: https://git.openjdk.java.net/jol/pull/19/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jol&pr=19&range=00
  Stats: 27 lines in 1 file changed: 27 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jol/pull/19.diff
  Fetch: git fetch https://git.openjdk.java.net/jol pull/19/head:pull/19

PR: https://git.openjdk.java.net/jol/pull/19