[WIP] Codegen partial classes. Implements #278

[SteveSandersonMS]

Although this works fine at compile time and runtime, it doesn't work correctly at design time in VS:

I'm unsure why partial classes from the same project aren't included in the design-time compilation that VS is getting these errors from, especially given that things like @inherits SomeClass works fine.

@rynowak, do you know why this is, and if there's any straightforward thing we can do in this repo to fix it?

Not urgent! This doesn't need to be included in 0.1.0.

[rynowak]

Ah, I see.

The problem with this is that the editor code is Fake. It mangles the class name so that it doesn't conflict with the file on disk. I'm sad that we can't do this trivially 😢

I have a few ideas for how to fix it, but they will be more involved.

[danroth27]

#278

[SteveSandersonMS]

@rynowak Since this PR is relatively simple, and the work to really enable partial might be a while off, I think I'll just close this and then we can reimplement/rediscover it later. Please reopen if you disagree.