[Breaking draft] Tweak block::Generator to support fast-key-erasure

[dhardy]

• Added a CHANGELOG.md entry

Summary

Tweak the block::Generator trait to support fast-key-erasure: RustCrypto/stream-ciphers#578

Also removes BlockRng::{reconstruct, remaining_results} since to my knowledge these are only used by the (mostly abandoned) Isaac generators.

Motivation

It turns out we can fit a low-overhead fast-key-erasure generator into our existing ecosystem with only some small tweaks here. (Larger changes to the block code may be warranted; this is more a proof-of-concept with minimal changes.)

Details

• Add associated type Generator::Word. Note that we require (without trait bound) that Generator::Output is [Word; N] for some N and that N is not currently an associated constant; ideally we'd add N and remove Output but this requires improved const generics support IIRC.
• Add fn Generator::erase. Note that this does a different job to drop: erase is used for fast-erasure (only, and without feature requirements) while drop is used to zeroize the buffer on drop (if that feature is enabled).
• Make fn Generator::generate return the index of the first available value in the buffer.