fix build breakage due to protolude `toS` changes
fix build breakage due to protolude `toS` changes

Protolude 0.3.0 switched to a different version of `toS` that is
non-partial and so removed a bunch of cases that `toS` supported.

This PR switches to the old `toS` that has been left in the Conv
module.

jml/haskell-spake2