Add index key for entities to improve use case performance

## Description
Add API can set key for entities, and use the key fast find entities
"specs::prelude::WorldExt" add function
fn find_entities(&self, key: T) -> Read
and add
fn create_entity_by_key(&mut self, key: T) -> EntityBuilder

## Motivation
In open source game "veloren"
I usually see the code.

```rust
let entity_opt = (&ecs.entities(), &ecs.read_storage::())
.join()
.find(|(_, player)| player.alias == player_alias)
.map(|(entity, _)| entity);
```

Basically, any query from player will loop all entities to find alias equal query name.
If ecs has 10000 players. the design include performance bottleneck.

## Drawbacks

* Is it a breaking change?
no.
* Can it impact performance, learnability, etc?
to improve performance for search specific entitie

## Unresolved questions
I just learn this project. I think author group will give me a best solution.

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