Add EntityMap::clear

[Shatur]

Objective

If you use EntityMap to map entities over network (https://github.com/lifescapegame/bevy_replicon) you need to reset it sometimes, but keep allocated memory for reuse.

Solution

• Add clear method.

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Changelog

Added

• EntityMap::clear.

[mockersf]

instead of adding all methods of HashMap to EntityMap, would it make more sense to impl Deref to HashMap?

[Shatur]

It will require adding DerefMut, but I guess it does makes sense.
Will convenient methods like get by value override DerefMut methods? Should we keep them?

[Shatur]

@mockersf what if we move world_scope to EntityMapper and just use plain HashMap everywhere?
If I understand correctly, EntityMap was created for better error handling, but we no longer have error handling after EntityMapper.

[alice-i-cecile]

@Shatur this code is fine (and well-motivated!), but the questions raised above are sensible. What's your preferred way to proceed:

1. Merge this PR as is, and split out the follow-up into a new issue / PR?
2. Close this PR, and just move directly to the final solution.

I think my preference is for 1, but I'll defer to you here.

[Shatur]

I would go with 1, it's a very small change.
And I will open a follow-up issue to discuss EntityMap.