chacha20 - rng - rand_core preview, minor set_stream() change and get_block_pos() change

[nstilt1]

It does not build because it requires one change to rand_core#44, and the Cargo.toml needs to be updated.

Changes

• Update rand_core to a preview from Revise index / reset / skip methods rust-random/rand_core#44
  • I need to change the Cargo.toml once that PR makes it through
• Update set_stream() to reset the block_pos + index to 0. Before, calling set_stream() would basically behave like this:

fn set_stream(stream_id: S) {
  self.stream = S.into();
  if self.index != 64 {
    // generate_and_set is called, resulting in
    self.block_pos += 4;
    self.index = self.index; // no change to the index, but change to the block_pos
  }
}

• • I had a test that had repeated set_stream() calls and the only way to replicate it was to manually find the word_pos that resulted from each call. We can remove this test or at the very least reduce the test, since I had to add more assertions to figure out what was going on in the previous implementation.
• Update get_block_pos() to return the current block_pos instead of returning block_pos + 4 due to generate() calls incrementing the pos by 4.
  • This is not a breaking change since get_block_pos() was not implemented for rand_chacha, but it is a breaking change for anyone who was using get_block_pos() from the prerelease versions.
• Update docs to explain that set_stream() nullifies any calls made to set_word_pos() or set_block_pos().

Regarding #438

[baloo]

NIT:
a way to make it work for everyone (and CI) is to put a patch section in the workspace's Cargo.toml instead:

[patch.crates-io]
# https://github.com/rust-random/rand_core/pull/44
rand_core = { git = "https://github.com/rust-random/rand_core.git", branch = "push-xzmlvzwurnrl" }