Add edge cases to Gen.choose[BigInt]

[DirkToewe]

The following property test should clearly fail:

object ScalaCheckCheck extends Properties("Gen.choose[BigInt]"):

  override def overrideParameters( params: Parameters ): Parameters
    = params.withMinSuccessfulTests(1_000_000)

  property("covers all bit lengths") = forAll(
    Gen.choose[BigInt](0, BigInt(1) << 64)
  )( _.bitLength > 32 )

end ScalaCheckCheck

But since Gen.choose[BigInt] samples uniformly, the chances of the test above (correctly) failing are less than 3 in 10_000.
The documentation does not mandate that Gen.choose[BigInt] samples uniformly, so I propose adding the following edge cases and sampling methods to Choose[BigInt]:

1. {0, ±1, ±2, ± Byte.MaxValue ± 1, ± Short.MaxValue ± 1, ...} (provided they are in range)
2. Randomly chosen small primes or probable primes
3. Products of 2 or 3 small/probable primes
4. Random Powers of 2
5. "Sparse" BigInt values with uniformly random bitLength and uniformly random number of 1-bits

If these additions are acceptable, I would be willing to work on an according PR.