improvement(generate:typetests,list): Only write output file when contents differs

[tylerbutler]

Adds a utility function to write to files only if the contents you're writing is different from what's already there. This function is used in the type test generation and flub list to ensure we only write changes when needed.

This function may be useful for other commands as well, but those changes are left for separate PRs.

I also opportunistically replaced some synchronous function calls with async counterparts.

[CraigMacomber]

I think the motive I had for suggesting this work (avoid needless file churn as it could invalidate incremental build state) was flawed for multiple reasons:

1. We don't use modification time of inputs vs outputs to do our incremental build
2. If we did use the input vs output build times, tasks which consume other outputs could end up forever dirty due to this change, which is what I was thinking it might fix.

In conclusion: If I was right about how our build worked (and I wasn't) this would be really bad. Since I don't actually know how this part of our build works, someone else should probably make the judgement on if we want to do this.

[jason-ha]

I still think it is a better idea to understand the root trouble with .done tracking before making things more complicated.

[jason-ha]

What happens if the file doesn't exist yet? I would guess it throws, but in that case we want an explicit write.

[jason-ha]

Can we add a note about performance improvements?
This helper fully reads and closes a file before writing it (if needed).
In the common case that writing is needed, it will be more efficient to open the file for read+write to start and keep it open until any writing is dealt with.