(fixup) Move the HChaCha initialization into expand().
(fixup) Move the HChaCha initialization into expand().

This brings the code more inline with that in pull request #230.

PeterReid/rust-crypto,PeterReid/rust-crypto,tempbottle/rust-crypto,DaGenix/rust-crypto,mseri/rust-crypto,tempbottle/rust-crypto,thinkingfish/rust-crypto,canndrew/rust-crypto,andreasots/rust-crypto,canndrew/rust-crypto,nmathewson/rust-crypto,mseri/rust-crypto,andreasots/rust-crypto,thinkingfish/rust-crypto,nmathewson/rust-crypto,DaGenix/rust-crypto