require haskell-src-exts-1.14.0

the old code was working fine with haskell-src-exts-1.13.5, but fails
with haskell-src-exts-1.14.0;
the new code is working fine with haskell-src-exts-1.14.0, but fails
with haskell-src-exts-1.13.5;

we have to make a choice of supporting one or the other, and we choose
the future.