Add RIPEMD160 implementation to `bitcoinj-base` module

[msgilligan]

We are trying to reduce our (direct) dependencies on Bouncy Castle. In particular, see Issue #3912.

A RIPEMD160 implementation is small, relatively simple and is only used to hash already hashed pubkeys, so I think it is a reasonable candidate for inclusion directly in bitcoinj-base.

For reference we could look at the Bouncy Castle implementation or the implementation in nayuki/Bitcoin-Cryptography-Library:

• Ripemd160.java
• Ripemd160Test.java