add to_flat, from_flat, like, and better handling for existing arrays / groups

[d-v-b]

• ArraySpec and GroupSpec now have a like() method for doing parametrized comparisons with other objects
• ArraySpec and GroupSpec can be created from flat representations of trees in the form of dict[str, ArraySpec | GroupSpec] (addresses add validation logic for relative paths #23, names with the / character in them #24)
• x.to_zarr now returns an Array or Group if a pre-existing array or group exists, as long as that array or group is structurally identical to x (addresses make to_zarr more subtle when encountering existing stuff #14)
• GroupSpec.members is now nullable
• Group.from_zarr takes a depth parameter, which determines how deep into the hierarchy to go. GroupSpec instances with members that were not examined due to this parameter will have members set to None (addresses Limit recursion in GroupSpec.from_zarr #2)