Expose label polymorphism via an opt-in feature

[Marwes]

To let type signatures take advantage of label polymorphism without breaking backwards compatibility we will add an opt-in compilation option that tells the compiler to use the label polymorphic function signatures.

How should the option be enabled

• Via an explicit option at the command line or GUI?
• Should we enable this option if the label syntax (.mylabel) is used in the script? ( Add a syntax to describe label values #4927 )
• Something like javscripts "use strict" in the script itself?
• If the script uses the label syntax, auto enable the feature
• Explicit Flux major versions

Since the feature changes the pre-compiled standard library we must either compile it twice, or ensure that we retain both signatures in one pre-compilation and information on how to select which (the former is easier, but less efficient).

(Based on the comment #4776 (comment) )

DOD

• When compiling we can select either the old or the new function signatures to use when typehecking

[github-actions]

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