libsnapshot: Add Sequence Ops

This adds the ability to write sequence ops. A Sequence op is a list of
block values in the new image. These ops may have dependencies on data
that is overwritten by future ops, so must be merged in a specific
order, regardless of their order in the cow file.

This patch only introduces the operation. The following patches will
actually make use of it.

Bug: 177104308
Test: Builds
Change-Id: I9162b49e5061079416ac9e5661c8b1245298044b