Expose NotFound as a specific StoreError

[rnewman]

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At present a missing key in a .delete call returns StoreError::LmdbError(lmdb::error::Error::NotFound). It is necessary to handle this case specifically in order to implement delete-if-present, which means that consumers need to take a dependency on lmdb-rkv.

One solution for this is to handle NotFound specially in rkv's own interface. I'm open to other suggestions.

[dfoxfranke]

+1. I'd go so far as to say that delete-if-present is the semantics almost everybody wants, and trying to delete a non-existent key should not be considered an error. I propose changing the return type of .delete() to be Result<bool, StoreError>, where the Ok variant is true if the key existed and false if it didn't.