Should running the codegen be part of the build process?

[Boddlnagg]

We need to figure out whether running the code generator should be part of building the library. If so, we should have a Cargo build script to run it as part of building winrt-rust.

So far, I'm inclined to say that we should have gen.rs checked into the repository and distribute it that way. This has the following advantages:

• We can generate it from the most recent version of the winmd files. I think that those are backwards compatible, so you can always compile against the latest version and if you use a feature that is not available on your local OS version, it will fail at runtime. (This can also be considered a disadvantage: you will get compile-time errors when you first run codegen from your local winmds, which is arguably better than runtime errors).
• You don't need a C# compiler and .NET runtime to build the project (might be good for cross-compilation from Linux if that ever becomes a thing)