join and rename preconditions removing ambiguity, `on` now optional

`join` and `rename` both had the ability to introduce some ambiguity, whether
by renaming two fields to the same alias (which one should use that alias?) or
by joining two fields with the same alias (which one should use it?). Some
preconditions have been added to warn against some ambiguous use cases
(although I have just now realised the rename precondition is insufficient, so
a new commit will be coming soon).

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