Thorough test suite

[dw]

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The current tests are a joke. Needs at least:

• Full set of tests written for py-lmdb (py-lmdb #21)
• Full set of tests for key encoder.
• Full set of tests for core interface.
• Full set of generic tests for engine implementations.
• Full set of tests for meta interface.
• Full set of tests for iterators.
• Smoke test using real database to detect accidental format incompatibilities

All edge cases: reverse iteration, prefixes ending in 0xff, etc etc etc