Bernoulli constructor

[dhardy]

In rand_distr we switched all constructors to return a Result unless they couldn't fail to avoid having to panic.

There is one distribution that didn't get updated: Bernoulli, which remains in Rand proper.

Should we make this return a result? For:

• it's consistent
• it avoids a panic

Against:

• it's a breaking change
• it's less convenient (though we still have Rng::gen_bool for convenience)

I think probably we should, but would like to hear opinions?

[vks]

We could also add a new, non-panicking constructor and deprecate the old one.

(I think I would prefer a breaking change to rand though. I don't expect there is so much code using Bernoulli directly.)