Replace SeedableRng impl for SmallRng with inherent fns

[dhardy]

Fixes #1285 by:

• Remove SeedableRng impl for SmallRng. Use inherent fns instead.
• Fix Seed type for StdRng (the only practical effect is to make a possible future breaking change to the API more obvious).

Loss of functionality:

• SmallRng can no longer be used in generics requiring a SeedableRng bound. As such, it can also no longer be reseeded from generic code. Is this an issue?
• There is no public SmallRng::from_seed function. This is deliberate since most useful uses require a portable RNG.

[dhardy]

Some tests failed because SmallRng::from_entropy is missing. But do we want this anyway? Instead, I just modified the benchmarks to use <$R>::from_rng(rand::thread_rng()).unwrap()

Additional change: make fn SmallRng::from_thread_rng infallible, since it appears that the only possible failure is a panic when initializing thread_rng.

Aside: if we could bring SeedableRng into rand proper, we could add from_thread_rng as a standard addition..