Consider switching the thread RNG to OsRng

[pcwalton]

I think we should not be encouraging ISAAC. The documentation even cautions against its use.

I know the compiler will get slow if you switch away from ISAAC, but we should just manually use ISAAC in the compiler if we really need that. Userspace CSPRNGs are bad.

[zackw]

ChaCha instead maybe?

[arthurprs]

Wouldn't that be order of magnitudes slower?

[dhardy]

I don't think switching thread_rng to OsRng is the right choice (it's been discussed; thread_rng or rather an equivalent is used for seeding HashMap's hash function and is designed as a fast, reasonably strong generator).

On the other hand we may increase thread_rng to full crypto strength: dhardy#53