yambs should know when to take no action for regeneration

[vkottler]

You should be able to invoke mbs as a means to just pass through to ninja.

[vkottler]

Maybe not writing files if their contents don't change would be the thing to do? Anything else beyond that?

[vkottler]

Depends on: vkottler/vcorelib#131.

[vkottler]

This is next up.

Need to come up with some interface so that if the contents being written to a file aren't changing, we don't unnecessarily update the file metadata (triggering some build systems).