Minimal extension for ,. syntax

[yitzchak]

We have just added specializations of the wrap-in-quote... generics to Clasp's eclector-client. Clasp does have destructive splicing (as rare as that is) but ,. gets turned into ,@ by Eclector,

This PR is an idea for a minimal extension to enable destructive splicing without changing the signature of wrap-in-unquote-splicing and making the default for wrap-in-unquote-nsplicing be the appropriate specialization of wrap-in-unquote-splicing.

Obviously, error messages, tests, etc. could be updated.

FYI @Bike