[CDEC-337] Fix and export SafeSigner generator
[CDEC-337] Fix and export SafeSigner generator

The SafeSigner generator didn't make use of both of its data type's constructors and it also wasn't exported.

input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl