[scudo][standalone] Introduce the chunk header
... 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
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