This is part of the effort to reduce Courgette's peak memory. Main changes:
- Moving Label to image_utils.h, and change Label::count_ from int to int32.
- Adding utility class ConsecutiveRangeVisitor, with tests.
- Adding LabelManager, with tests.
The new code is not yet used in production.

Review URL: https://codereview.chromium.org/1491703003

Cr-Commit-Position: refs/heads/master@{#363688}