implement equal? using ExistentialQuantification

added the ExistentialQuantification extension, defined a higher minimum version of cabal, and defined Haskell98 as the preferred version of Alonzo Church, probably

with this update, alonzo can handle the following:

    ./dist/build/alonzo/alonzo "(cons '(this is) 'test))"  // ((this is).test)
    ./dist/build/alonzo/alonzo "(eqv? 1 3)"                // #f
    ./dist/build/alonzo/alonzo "(eqv? 3 3)"                // #t
    ./dist/build/alonzo/alonzo "(eqv? 'atom 'atom)"                // #t