Update Excalidraw Design

As code is written the design may need tweaked. This issue tracks the changes that need to happen to the Excalidraw document. It's probably easier to just make all these changes at once.

• Move Buffer::reallocate to Allocator
  • Allocation makes buffers, attempting to reallocate in the buffer leads to weird scenarios where you ask a TABuffer to reallocate itself as say a TAMM buffer. Thus I think reallocation needs to move up a level to Allocator::reallocate(Buffer&).
• Similarly I think reshape may need moved to.
• Revisit expression layer. As it's mocked-up I think we need to actually implement two expression layers (one for the buffer and one for TW proper). I think we can get by with just one if we drop the layer associated with the buffer.
• add reshape(Buffer&) function to Shape

Start with the Excalidraw document and review the listed Buffer, Allocator, Shape, and expression-layer concepts. Resolve whether reallocation and reshape belong at the allocator or shape level, and whether one or two expression layers are needed. Done means the design document reflects a settled architecture and all listed changes.