Generate random BigUints and BigInts
test the correct function (oops)
Incorporate feedback from huonw

- use identifiers with underscores to avoid unused variable warning
- implement on R: Rng instead of on R: RngUtil
- bugfix: zero BigInts were being generated twice as often as any
  other number
- test that gen_biguint(0) always returns zero
Merge RandBigUint and RandBigInt into single trait
auto merge of #9007 : dcrewi/rust/random-bigints, r=huonw