Read detached crates #232
Description
We should be able to read detached crates, as defined by the 1.2 spec. Possibly this could set a ROCrate.is_detached flag.
Once the metadata is loaded it can be treated the same as an attached crate - it's only when reading/writing that we need to care about the difference.
Related to #231 - particularly if reading a crate in then writing it back out again.