Randomness: implementation and use getrandom for Wasm target

[Mec-iS]

I'm submitting a

• feature request.

Current Behaviour:

RealNumber::rand is not implemented. Floatnumber::rand use an external crate that cannot work on Wasm.

the use of random in fn rand() is ambiguous and may fail in some targets. Some decisions have to be taken about the usage of rand::Rng also. I suggest going for using getrandom as it applies to the most targets.

cc: @morenol

[Mec-iS]

related to #216

[morenol]

We use getrandom with js feature for WASM in browser target:

smartcore/Cargo.toml

Line 32 in 4cf7e4d

js = ["getrandom/js"]

I think that rand works without troubles for WASI target

[Mec-iS]

ok then we need to clear up the features:

default = ["serde", "wasi", "datasets"]  // datasets should also use gerandom
wasi = [..add deps ..]
...

optimally for wasi we should be able to do also no_std?

[morenol]

I think that we can get ride of that issue using

        let mut rng = get_rng_impl(None);

instead of

let mut rng = rand::thread_rng();

[Mec-iS]

ok but then shouldn't we use getrandom instead of rand for the reasons above?
get_rng_impl(None) uses rand.

[morenol]

Not completely sure if we need this. Rand is the high level API and the README of that project says:

This is a low-level API. Most users should prefer using high-level random-number library like [rand](https://crates.io/crates/rand).

[Mec-iS]

we need lower-level implementation for default feature, so in the future we can consider getting closer to no_std.

[Mec-iS]

This is not fixed yet