consider `base64` for `r*`

[joshmoore]

          We may want to consider `base64` as it packs better, is JSON, compatible, and [there is some precedent from v2]( https://zarr-specs.readthedocs.io/en/latest/v2/v2.0.html#fill-value-encoding )

Originally posted by @jakirkham in #330 (comment)