Stable buffer

[matthewhammer]

Extends #516 with an example stable-region-based data structure, for "stable buffers" that

1. grow one blob at a time and
2. blob can reappear within a buffer later without taking more space (e.g., for storing elements in a system that recognizes duplicates somehow, which is outside of the scope of this structure for now).

For simplicity, the structure only stores Blobs. It could easily be extended to store other primitive types too.