Add Hash160 algorithm

[jmjatlanta]

This algorithm is used by Bitcoin's BSIP199 for Hashed Time Lock Contracts. This is part of BitShares BSIP64

I attempted to remove the impl, as I am not using it. Unfortunately things will not compile without it, and I am not familiar with the templates that answer "why" this is the case. I can leave it as is, move the SHA256 stuff into the impl, or spend more time digging into the "why" and see what I can do.

Suggestions appreciated.

[pmconrad]

I attempted to remove the impl, as I am not using it.

AFAICS the only place where encoder::impl is used is here: https://github.com/bitshares/bitshares-fc/pull/143/files#diff-c871c59a37683d8807f37150fab164eeR68
my in turn doesn't seem to be used anywhere. fc::fwd only reserves sufficient space for impl - the idea is that it can be stack-allocated without knowing the full declaration of impl.
Did I miss anything?

[jmjatlanta]

This PR will help with the HTLC improvements BSIP that will hopefully be accepted.

https://github.com/bitshares/bsips/blob/master/bsip-0064.md

[abitmore]

There are new changes on the ripemd160 files after you created your fork, please apply the same changes to your new class. Thanks.

[jmjatlanta]

There are new changes on the ripemd160 files after you created your fork, please apply the same changes to your new class. Thanks.

Done.

[abitmore]

It seems your encoder implementation lacks of a reset() function, which means callers need to multiple encoder instances for encoding multiple items.

[jmjatlanta]

It seems your encoder implementation lacks of a reset() function, which means callers need to multiple encoder instances for encoding multiple items.

Was in header, but forgot to implement. Nice catch.

[abitmore]

I don't know why my is needed for compile, and don't want to spend more time researching it at this moment. To avoid unexpected trouble, please update your code to be inline with other classes, move the implementation details to the impl class. Thanks.

[abitmore]

Thanks.