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Only allow keys of the form `/[0-9A-Z+-_=]`. That is, upper-case alphanumeric
keys in the root namespace (plus some special characters).

Why? We don't encode keys before writing them to the filesystem. This change
ensures that:

1. Case sensitivity doesn't matter because we only allow upper-case keys.
2. Path separators and special characters doesn't matter.

For context, go-ipfs only uses flatfs for storing blocks. Every block CID is
encoded as uppercase alphanumeric text (specifically, uppercase base32).

We could be less restrictive, but this is safer and easier to understand.
Unfortunately, we _can't_ allow mixed case (Windows) and can't allow lowercase
because we're already using uppercase keys.

fixes #23