Remove use of thread_rng in common

[maurges]

Another patch of bringing this library a bit closer to no_std. Now rand crate is used with no_std for library, but with std for tests.

This only touches the functions in "common" module, so does not affect public api.

[maurges]

Good idea. Although doesn't actually matter for dependency size, since num-bigint requires rand anyway.

[maurges]

@mikelodder7 can you take a look at the new changes? I believe your concerns were addressed by them.

[maurges]

Thanks!