Revise index / reset / skip methods

[dhardy]

• Added a CHANGELOG.md entry

Summary

Make fn index private; add fn word_offset as a pub replacement.

Remove fn reset; rename fn generate_and_set to reset_and_skip.

Motivation

The old fn index was easy to mis-use: RustCrypto/stream-ciphers#438 (comment). I don't think this was the first time.

We also don't need two "reset" methods; it should be equivalent to call reset_and_skip(0).

@nstilt1 please review.

[dhardy]

From the other PR and patch I take it that @nstilt1 finds this acceptable.