SuffixHandler: add Suffix newtype

This commit adds a Suffix newtype to describe suffixes as handled
by suffix handlers & preprocessors, and changes the PPSuffixHandler
type definition to use it.

It also moves some type definitions from Distribution.Simple.PreProcess
to the new module Distribution.Simple.PreProcess.Types.

As this commit changes the definition of PPSuffixHandler, it will
break custom Setup scripts which use the 'hookedPreProcessors'
functionality.