Variant of Query::get that takes an EntryRef

I propose adding a variant of the three `Query::get` functions, `Query::get_from_entry` or the like, that takes an `EntryRef`/`EntryMut` instead of a `World` and `Entity`. If you want to use several queries on the same entity, then it is presumably faster to get the entry once and reuse it? Depending on what you think is better, these new functions might either be added alongside the current `Query::get` functions, or the existing functions be modified to work this way instead.