[scudo][standalone] Introduce the combined allocator

Summary:
The Combined allocator hold together all the other components, and
provides a memory allocator interface based on various template
parameters. This will be in turn used by "wrappers" that will provide
the standard C and C++ memory allocation functions, but can be
used as is as well.

This doesn't depart significantly from the current Scudo implementation
except for a few details:
- Quarantine batches are now protected by a header a well;
- an Allocator instance has its own TSD registry, as opposed to a
  static one for everybody;
- a function to iterate over busy chunks has been added, for Android
  purposes;

This also adds the associated tests, and a few default configurations
for several platforms, that will likely be further tuned later on.

Reviewers: morehouse, hctim, eugenis, vitalybuka

Reviewed By: morehouse

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

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D63231

llvm-svn: 363569