Made arrays strict by default; please merge to release

* Added parameter `array-strictness', default #t

* Added `array-default-strict!' and `array-default-strict', which act
  like the functions without "default" in the name when
  `array-strictness' is #t; otherwise they do nothing

* Lots of small changes to existing array functions, mostly to ensure
  computations are done using nonstrict arrays, but return values are
  strict when `array-strictness' is #t

* Added strictness tests

* Added tests to ensure untyped code can use `math/array'

* Rewrote `array-map' exported to untyped code using untyped Racket

* Rearranged a lot of `math/array' documentation