RenderBuffers that can store contents indefinitely

Rather than storing a temporary "delta" of changes to be flushed to the terminal, it would be useful to allow a RenderBuffer to store long-lived content after it is flushed. This would let them be used as a form of "canvas", storing a persistent image.

A RenderBuffer can already do that now by just using the `tickit_renderbuffer_blit()` operation to copy it to another. However, there is no garbage collect mechanism at present to reclaim the memory used by text strings that have since been overwritten. Essentially then, this blueprint requires adding such a mechanism.