Rename date function

[opoudjis]

date function overlaps with the date label that is used in ISO, and results in clashing. Need to (a) rename the function (it's just less disruptive to), and (b) preemptively stop labels from being reassigned as methods, and crash isodoc-i18n, if there is such a clash, it indicates poor input.

[ronaldtse]

What's a "date" label? How is it different from the "date" command?

[opoudjis]

Date is the string "date", internationalised. It is different from the function, and it is a bug that they are aliased.

There is no need to abort, I just need to not assign labels as function aliases, when there is a conflicting function already in place. I am allowing @i18n.terms_and_definitions as an alias of @i18n.labels["terms_and_definitions"], and I simply don't create @i18n.date as an alias of @i18n.labels["date"], because @i18n.date() is already defined.

[opoudjis]

Confirmed with date:[] document in ISO, where the clash arises.