Replace global-scope helpers finToNat and finToInteger with functions whose type signatures match the names.
Add FinZAbsurd and FinZElim.
Replace Prelude.Vect.take and Prelude.Vect.drop with versions of a saner type signature.
Replace Prelude.Vect.intersperse with a saner-typed version. Make intersperse' a proper helper function (non-exported).
Merge branch 'prelude_cleanup' of https://github.com/sjdrodge/Idris-dev into sjdrodge-prelude_cleanup