Implement GenericWriteStorage::get_mut_or_default

[JaniM]

Description

Implement GenericWriteStorage::get_mut_or_default, which either fetches an existing component, or calls Default::default on the component type, inserts the newly created struct in to the storage, and returns a mutable reference to it.

This can never fail.

Motivation

I found myself writing a lot of code along the lines of

if let Some(stun) = data.stun.get_mut(entity) {
    stun.time += 1;
} else {
    data.stun.insert(entity, Stun { time: 1 }).unwrap();
}

Verbose, and extremely repetitive. The new function offers a shorthand:

data.stun.get_mut_or_default(entity).time += 1;

Drawbacks

An extra function to maintain. However, it builds on the existing methods of GenericWriteStorage, so the maintenance impact should be near zero.`

Unresolved questions

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Opened a PR for this.