Change Hasher and HMAC APIs closer to std::hash model

- Implement Clone and std::io::Writer.
- Reduce the API to write() and finish(). Contrary to std::hash, finish() resets the hasher immediately.
- Add hmac::hmac() convenience fn.
- Replace hash::evpmd() with HashType methods.
- Add assertions as a crude check for failed calls into openssl.
- Add examples and some tests.

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