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This CL adds ensemble patching (AKA Layer 3 patching) to Zucchini, where
patches are generated and applied on "old" and "new" archives that
contain uncompressed executable "elements", which need to be detected
and matched first.
- HeuristicEnsembleMatcher performs element detection and matching.
  - Resulting matched "elements" undergo architecture-specific patching
    (AKA Layer 2 patching), but on failure, raw patching (AKA Layer 1
    patching) is used.
- "Gaps" are contents in archives that are not elements. These use
   raw patching, and are computed separately from element patching to
   reduce peak memory (since these require suffix array of the entire
   "old" archive).
- In the patch format, "gaps" are represented as "no-op" elements,
  i.e., raw patches without references.
- This CL focuses on Zucchini-gen, since Zucchini-apply already supports
  Ensemble patch application.
- After this CL, Zucchini will be equally usable as Courgette.

Bug: 729154
Change-Id: Id828186cda74885ee6992b17f641e491efe79c40
Reviewed-on: https://chromium-review.googlesource.com/891489
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533734}