BLOCKER: Clarify the role of entry_index, maybe rename

[cuviper]

Before we cut any new release, we should resolve the questions on the new entry_index.

@bluss #115 (comment) :

Wait - this method seems underspecified and surprising? Both the PR and the doc comment needs to explain in detail what it does.

The method should not be called entry_* when it is more similar to get_full than to entry. I can guess that the motivation for adding this is performance, just having a simpler return type than get_full, is not enough to warrant inclusion.

@cuviper #115 (comment) :

Yes, the main advantage I see is to avoid the indexing operation in get_full, which seems reasonable to me. The author originally called it get_index, which might be a better name, but that method already exists for using an index to get the key-value.

cc @Thermatix