Node name & path updates from ZEP 1 review

[jstriebel]

This applies feedback from the ZEP1 review in PR #149 related to node names and paths:

• removes 255 character limit for paths
• adds note about restrictions (case insensitivity, key length limit) on underlying stores (Windows, S3, MacOS)
• removes the /root prefix:
  • meta/root.array.json is now meta/array.json
  • meta/root/foo/bar.group.json is now meta/foo/bar.group.json

fixes #77

Note for reviewers: More changes about those topics are likely to come and discussed in different issues. A good overview of those should be #178.

[jstriebel]

I'd love to merge this soon to avoid conflicts and to have the feedback incorporated into https://zarr-specs.readthedocs.io/en/latest/core/v3.0.html, it would be great if one of you could look into those changes @rabernat @joshmoore @alimanfoo @jakirkham @jbms.