creating release tag v0.7.

Relevant changes (since v0.6):
- Faster scalar backend and AVX-512 backend
- Add tests/bench, which measures median cycles per byte (CPB) for a variety of functions and message lengths
- Add `SHA3_BACKEND` make argument and define to control backend at compile-time
- Add `sha3_backend()` function to query backend at run-time
- Add experimental `neon` and `diet-neon` backends.  Currently slower than scalar and disabled by default.
- Add "Backends" section and "Benchmarks" section to documentation
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