C# future simple codegen

[yowl]

This PR adds enough code gen to support the simple-future wit runtime test. As for the async PR, this is pretty much the minimum PR in terms of future support. I've not tackled the typed canonical methods except to add a "void" implementation which is hard coded as the one to use.

Have followed the c test cases rather than the rust ones.

Also changed Export and Import in namespaces to be uppercase and moved resources and other methods to the appropriate import or export class. Some types are still produced from the import side, and have introduced a concept of a bidirectional type (enum, flags) that sit above the import/export split.

The current codegen produced is at https://github.com/yowl/wit-bindgen-simple-future

[yowl]

cc @pavelsavara @dicej @jsturtevant

[dicej]

Thanks for doing this, @yowl! I'm planning to review it by the end of the week.