Update to extra-1.7.9

1.7.9, released 2020-12-19
    #72, add pureIf
    #73, make takeEnd, dropEnd, splitAtEnd: return immediately if i <= 0
    #71, add compareLength and comparingLength
1.7.8, released 2020-09-12
    #68, make sure Data.Foldable.Extra is exposed
1.7.7, released 2020-08-25
    #67, defer to System.IO readFile', hGetContents' in GHC 9.0
1.7.6, released 2020-08-21
    #66, add lots of functions to Data.Foldable.Extra (anyM etc)
1.7.5, released 2020-08-12
    #65, add Data.Foldable.Extra
    #65, add sum', product', sumOn' and productOn'
1.7.4, released 2020-07-15
    #59, add whileJustM and untilJustM
    #61, optimise nubOrd (10% or so)
    Add first3, second3, third3
1.7.3, released 2020-05-30
    #58, add disjointOrd and disjointOrdBy
1.7.2, released 2020-05-25
    #56, add zipWithLongest
    #57, make duration in MonadIO
    Simplify and optimise Barrier
    Mark modules that are empty as DEPRECATED
    Remove support for GHC 7.10
1.7.1, released 2020-03-10
    Add NOINLINE to errorIO to work around a GHC 8.4 bug
1.7, released 2020-03-05
*  #40, delete deprecated function for
*  zipFrom now truncates lists, rather than error, just like zip
1.6.21, released 2020-03-02
    #54, deprecate nubOn since its O(n^2). Use nubOrdOn
    #53, add some nub functions to NonEmpty
1.6.20, released 2020-02-16
    Add firstM, secondM
1.6.19, released 2020-02-11
    #50, add headDef, lastDef, and dropEnd1

(pho)