add ieee754 next* functions to math_funs
add ieee754 next* functions to math_funs

Reviewers: austin, simonmar, trofi, bgamari

Reviewed By: bgamari

Subscribers: mpickering, thomie

Differential Revision: https://phabricator.haskell.org/D2671

GHC Trac Issues: #12802

shlevy/ghc,olsner/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,mettekou/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,olsner/ghc,sdiehl/ghc,sdiehl/ghc,mettekou/ghc,mettekou/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,mettekou/ghc,mettekou/ghc,ezyang/ghc,olsner/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,ezyang/ghc,olsner/ghc,ezyang/ghc