Add support for every database flags

[Kerollmops]

This Pull Request is based on #197.

It brings the support of the DUP_SORT, DUP_FIXED, REVERSE_KEY, INTEGER_KEY, INTEGER_DUP, and REVERSE_DUP flags.

I removed the Database::append method with the more generic Database::put_with_flags one. This new method can use the NO_DUP_DATA, NO_OVERWRITE, APPEND, and APPEND_DUP options. The same happened for all the Iterator::append methods replaced with Iterator::put_current_with_flags methods.

• Ensure a Database is always opened with the same flags.
• Implement Debug on the different iterators.
• Add three new type parameters to the multiple iterators, exposing their behavior.
  • Introduce the IterationMethod trait.
  • The three MoveThroughDuplicateValues, MoveBetweenKeys, and MoveOnCurrentKeyDuplicates Zero-sized types.
  • Make sure that moving on the first or last also moves on the last dup value.
• Add documentation example of the different flags:
  • For the DUP_SORT flag
  • For the DUP_FIXED flag
  • For the REVERSE_KEY flag
  • For the INTEGER_KEY flag
  • For the INTEGER_DUP flag
  • For the REVERSE_DUP flag

[mikedilger]

I'm hoping for some kind of delete_dup(key, value)

[Kerollmops]

Follow this toot instructions.

[Kerollmops]

As I am the only one available on the team and the CI seems happy, I will merge by hand without approval 🆘