implement rand(::Range{BigInt})

* Previously, calling e.g. rand(big(1:4)) caused a stack overflow,
  because rand(mt::MersenneTwister, r::AbstractArray) was calling
  itself recursively. This is fixed here, but a mecanism handling
  not-implemented types should be added.
* RandIntGen is renamed to RangeGeneratorInt.
* A type RangeGeneratorBigInt similar to RangeGeneratorInt (both
  subtypes of RangeGenerator) is introduced to handle rand on
  BigInt ranges. RangeGenerator is the generic constructor of such
  objects, taking a range as parameter.
  Note: two different versions are implemented depending on the
  GMP version (it is faster with version 6 on big ranges).
* BigInt tests from commit bf8c452112 are re-added.