impl ExclusiveSystemParam for SystemName

[stepancheg]

Objective

SystemName might be useful in systems which accept &mut World.

Solution

• impl ExclusiveSystemParam for SystemName
• move SystemName into a separate file, because it no longer belongs to a file which defines SystemParam
• add a test for new impl, and for existing impl

Changelog

• impl ExclusiveSystemParam for SystemName

[alice-i-cecile]

Good. I'm particularly glad that we're slowly splitting up that huge file.

[hymm]

This is a breaking change if you were importing SystemName from system_param::SyatemName

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[stepancheg]

@hymm it is not because SystemName is exported in mod system.

[stepancheg]

I guess somewhere out there there are tools to detect mechanical API changes automatically and correctly.

[alice-i-cecile]

Yeah, the tool in question would be https://crates.io/crates/cargo-semver-checks. Probably worth adding to our CI pipeline down the line.