Node name changes

[alimanfoo]

Following on from discussion on the community call today, this PR proposed two changes regarding the specification of node names:

• The spec now requires that stores are case sensitive. E.g., the operations set('foo', a) and set('FOO', b) will result in two distinct (key, value) pairs being stored. Subsequently get('foo') will return a and get('FOO') will return b.

• A note has been added stating our desire to support Unicode in node names and linking to the appropriate issue for comment.

xref #56, #57

[alimanfoo]

cc @Carreau

[jakirkham]

I think this came up on a previous call. On macOS, the filesystem is case insensitive by default (and this is the common case in the wild). Unfortunately changing a filesystem to be case sensitive is non-trivial as it requires reformatting. Also it may cause breakage in applications that have already assumed things are not case sensitive.

In practice, if neither FOO or foo existed and one of them was created, casing would match the one created (say FOO). However there would not be an option to create both as they would resolve to the same file or directory.

[jakirkham]

Should add I'm not saying we should make the choice based on macOS's behavior, which is admittedly odd. Just noting this will be a stumbling block for a potentially large set of users. Perhaps we can recommend users avoid creating a path that would conflict on case insensitive storage mediums? Though leave this in users hands.

[Carreau]

Closed in favor of #97 which is rebased to solve conflicts, but has the same content.