[code-reflection] Integrated: Decouple iface segment mapper allocation from Acclerator

[Gary Frost]

Currently we must pass an accelerator to create an iface buffer mapping

This does not make sense.  The allocation probably needs to come from the backend. 

So we decouple by creating a BufferAllocator interface which performs the task.

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Commit messages:
 - Decouple iface segment mapper allocation from Acclerator

Changes: https://git.openjdk.org/babylon/pull/138/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=138&range=00
  Stats: 90 lines in 16 files changed: 47 ins; 5 del; 38 mod
  Patch: https://git.openjdk.org/babylon/pull/138.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/138/head:pull/138

PR: https://git.openjdk.org/babylon/pull/138