Add mention of CSV in Formats#fromString.
Revert "Move Key to the public API."

This reverts commit 28c5415a83db967eebb00ce941e18b263d0e6aeb.
Rename spi.Key to spi.StorageKey.
Introduce public Key for RandomAccessDataset.

Note that this Key does *not* subclass Marker, since Marker is
now in the SPI, and we don't want SPI classes to leak into the
public API. Also, Key shouldn't have lots of internal-only methods
exposed that users don't need.
Remove DatasetRepositories#register from public API since it is an SPI mechanism.