request for comments: foreign calls (#20693)

I'd like it to be easier to extend GHC with support for new species of foreign calls. Existing support makes it easy to *replace* GHC's implementation of foreign calls but not to *extend* it. Details in #20693 (https://gitlab.haskell.org/ghc/ghc/-/issues/20693). I'd welcome comments, especially from anyone involved with GHCJS. Norman