[ issue 1283 ] Added type signatures
[ Utils.List ] Renamed mhead and mlast to headMay and lastMay

I took the names from the safe library (see
http://hackage.haskell.org/package/safe).
[ Utils.List ] Add headDef function.

From the safe library (see http://hackage.haskell.org/package/safe).
Merge branch 'maint-2.4.2'