[scudo][standalone] Introduce the chunk header

Summary:
... and its related functions.

The structure and its functionalities are identical to existing ones.
The header stores information on a `scudo::Chunk` to be able to detect
inconsitencies or potential corruption attempts. It is checksummed for
that purpose.

Reviewers: morehouse, eugenis, vitalybuka, hctim

Reviewed By: vitalybuka

Subscribers: mgorny, delcypher, jfb, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D61654

llvm-svn: 360290