array, some functions

   a = [ "a", "b", "c" ]
   a.index("b")        #=> 1
   a.index("z")        #=> nil
   a.index{|x|x=="b"}  #=> 1
This is an alias of find_index. 

   a = [ 11, 22, 33, 44 ]
   a.fetch(1)               #=> 22
   a.fetch(-1)              #=> 44
   a.fetch(4, 'cat')        #=> "cat"
   a.fetch(4) { |i| i*i }   #=> 16

   a = [ "q", "r", "s", "t" ]
   a.first    #=> "q"
   a.first(1) #=> ["q"]
   a.first(3) #=> ["q", "r", "s"]


   ar = [:a, :b, :c]
   >> ar.slice(0..-1)
   => [:a, :b, :c]
   >> ar.slice(0..1)
   => [:a, :b]

   [ "a", "b" ].concat( ["c", "d"] ) #=> [ "a", "b", "c", "d" ]

   ary = [1, 2, 4, 2]
   ary.count             # => 4
   ary.count(2)          # => 2
   ary.count{|x|x%2==0}  # => 3


   [ "a", "b", "c" ].join        #=> "abc"
   [ "a", "b", "c" ].join("-")   #=> "a-b-c"

   [ "w", "x", "y", "z" ].last   #=> "z"

   [ "a", "b", "c" ].reverse   #=> ["c", "b", "a"]

   a = [ "a", "b", "c" ]
   a.reverse_each {|x| print x, " " }
   produces:
   c b a

   a = [ "d", "a", "e", "c", "b" ]
   a.sort                    #=> ["a", "b", "c", "d", "e"]
   a.sort {|x,y| y <=> x }   #=> ["e", "d", "c", "b", "a"]

   a = [ "a", "a", "b", "b", "c" ]
   a.uniq   #=> ["a", "b", "c"]

   a.values_at(-1, -3, -5, -7)
   a.values_at(1..3, 2...5)