friction with our use of the `Buffer` class

[d-v-b]

This:

store = {"zarr.json": default_buffer_prototype().buffer.from_bytes(b'{"zarr_format": 3, "node_type": "group"})'}

Is worse than this:

store = {"zarr.json": b'{"zarr_format": 3, "node_type": "group"}'}

We could use the second option if we modeled the contents of stores with a protocol instead of a class like Buffer. I think this would be a big win for usability.