Allow to use `RandomState::new()` without any features

[a1phyr]

When building aHash without any features, the RandomState::new is disabled. I think it would be great to enable it in this case as in previous versions.

[tkaitchuck]

Yes. That would be desirable see discussion here:
rust-lang/hashbrown#207
Based on earlier discussion here:
#48

The question is how to do that "safely". Meaning that users don't make assumptions about DOS resistance that would not apply given the lack of both runtime and compile time randomization.

[tkaitchuck]

getrandom works on many but not all targets. (And those unsupported targets are more likely if we're talking about no-std builds.)

Obviously re-using the generated random data is a useful property too. Currently this is done via Lazy static. Atomics + Box might be an alternative, as could thread-local. Both have issues similar in that they aren't supported universally. For reference the standard library calls sys::hashmap_random_keys() which obviously is not available without std.

Previously the code in this case was using the history / order of memory offsets where maps were constructed as a (poor) source of randomness. This is certainly better than using fixed keys, because even if it is predictable at least accidentally self-inflicted dos attacks don't happen. But it can't be labeled as secure, so it seems like maybe it should go via a different method so it's not confused.