RFR: Loom checked stack chunk casts [v4]

[Stefan Karlsson]

> Replace raw oop to stackChunkOops casts with calls to a ::cast function which asserts that the oops is a stackChunkOop. This is similar to what we do for other types like *Klass, *CollectedHeap, *Thread, ...
> 
> Given that we assert when converting oops to stackChunkOops, I've also removed the redundant is_stackChunk asserts.

Stefan Karlsson has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:

 - Merge remote-tracking branch 'loom/fibers' into loom_checked_stack_chunk_casts
 - Allow nullptr in the cast function
 - Don't cast nullptr to oop
 - Remove redundant is_stackChunk asserts
 - Use cast function to convert from oop to stackChunkOop

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Changes: https://git.openjdk.java.net/loom/pull/119/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=119&range=03
  Stats: 64 lines in 10 files changed: 11 ins; 16 del; 37 mod
  Patch: https://git.openjdk.java.net/loom/pull/119.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/119/head:pull/119

PR: https://git.openjdk.java.net/loom/pull/119