Ideas for the first real-world usage of `n2o5`

[lynzrand]

Although n2o5 may be a pretty good idea on its own, it will lack the maturity for production use without some tools to try out for futher polishment first. This issue is for gathering ideas for the first real-world usage of it.

[lynzrand]

My current idea is that the first use case might be in build.rs scripts, so that C compilation and code generation can be incremental. This will put it in the same position as an in-process Makefile executor, allowing C projects to be more tightly co-developed with Rust files.

For this, we need the following features:

• Integration for cc-rs
• Easier graph generation