Make naming of "unnamed" relationships consistent

Previously anonymous relationships were named as `   UNNAMED<n>`, such that `<n>` was:

- For single length relationships with data: the character index immediately after the `[`;
- For single length relationships without data: the character index immediately after the first `-`; or
- For varlength relationships: the character index immediately after the end of the preceeding node (either the identifier or the closing brace).

This commit makes `<n>` consistently refer to the first character in the relationship declaration, being either `<` or `-`.