RFR: Garbage collectors explicitly transform stack chunk

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that makes garbage collectors explicitly transform stack chunks instead of relying on magic happening in `InstanceStackChunkKlass::copy_dis/conjoint`.

It also contains some refactoring suggested by @stefank in `Generation::promote`. Other than that it is a fairly straightforward change to replace `InstanceStackChunkKlass::copy_dis/conjoint` calls with explicit calls to `Copy::aligned_dis/conjoint_words` and `ContinuationGCSupport::transform_stack_chunk`.

This reduces code, and makes the code more understandable.

Test: tier1-5

Thanks,
  Thomas

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Commit messages:
 - undo assumption that stackchunks are not humongous
 - Clenaup
 - eriko "review" comments
 - Initial version

Changes: https://git.openjdk.java.net/loom/pull/116/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=116&range=00
  Stats: 133 lines in 18 files changed: 27 ins; 84 del; 22 mod
  Patch: https://git.openjdk.java.net/loom/pull/116.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/116/head:pull/116

PR: https://git.openjdk.java.net/loom/pull/116