Add Default for ThreadRng

[burdges]

Appears harmless. I'm unsure if this actually helps anything, especially since doing this for OsRng requires unwrap, but RandomState: Default though so..

[dhardy]

I guess this lets people derive(Default) if they want to do that. Not really sure whether including ThreadRng in a field is a good pattern, but I suspect it makes sense in some cases.

We opted to keep thread_rng#404 but I don't see any harm in adding this.

One suggestion: add brief documentation on this implementation and on fn thread_rng stating their equivalence.

[dhardy]

WASM fails us again 🙁

[burdges]

Oh? So thread_rng() works but not this? Odd.

[dhardy]

I suspect the test is failing on master too?

I restarted it: https://travis-ci.org/rust-random/rand/builds/459690582

[dhardy]

@burdges can you please rebase since #660 should fix the test.

[burdges]

We'll see if that works then.

I suppose if anyone needs OsRng: Default then they can wrap OsRng using whatever .unwrap() makes them happy. We might've delegation before that ever comes up. :)