RFR: Split out ContinuationEntry and ContinuationWrapper

[Coleen Phillimore]

Tested locally.  Mach5 too lame to test this.

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Commit messages:
 - Moved ContinuationWrapper out of continuation.cpp
 - Split ContinuationEntry out of Continuation

Changes: https://git.openjdk.java.net/loom/pull/143/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=143&range=00
  Stats: 1092 lines in 29 files changed: 638 ins; 430 del; 24 mod
  Patch: https://git.openjdk.java.net/loom/pull/143.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/143/head:pull/143

PR: https://git.openjdk.java.net/loom/pull/143