Allocator implementation API

Oveview

Some users will need to directly implement the MultiBufAllocator interface in order to provide allocation out of a particular region, provide particular allocation policy, fix chunks to some size (such as MTU size - header for socket implementations), or specify other custom behavior. These users will also need to understand and implement the ChunkRegionTracker API.

A simple implementation of a ChunkRegionTracker is provided, called HeaderChunkRegionTracker. It stores its Chunk and region metadata in a Allocator allocation alongside the data. The allocation process is synchronous, making this class suitable for testing. The allocated region or Chunk must not outlive the provided allocator.

Another ChunkRegionTracker specialization is the lightweight SingleChunkRegionTracker, which does not rely on Allocator and uses the provided memory view to create a single chunk. This is useful when a single Chunk is sufficient at no extra overhead. However, the user needs to own the provided memory and know when a new Chunk can be requested.

Classes
class	pw::multibuf::HeaderChunkRegionTracker
class	pw::multibuf::SingleChunkRegionTracker