contrib/anaphoric: More anaphoric macros added

Adding some more anaphoric macros 

* hy/contrib/anaphoric.hy: The following anaphoric macros have been
  added
  `ap-reject` : Opposite of ap-filter, yields the elements when a `pred`
  evaluates to false
  `ap-dotimes` : Execute body forms (possibly for side-effects) n times
  with `it` bound from 0 to n-1
  `ap-first` : return the first element that passes predicate
  `ap-last`  : return the last element that passes predicate
  `ap-reduce`: anaphoric form of reduce that allows `acc` and `it` to
  create a function that is applied over the list

* docs/contrib/anaphoric.rst: the above functions documented

* tests/__init__.py: updated to explicitly include tests for anaphoric
  macros