Review What "deprecate" Metadata Can Meaningfully be Applied To

[InsertCreativityHere]

Due to our non-existent metadata validation, the Slice compiler allows you to put ["deprecate"] on pretty much anything.
But, what this actually affects is very language dependent, and I think we should make it more consistent,
preferably by handling it more gracefully in more languages.

For example, in C++, ["deprecate"] only has an effect on operations and interfaces.
But I don't see a reason not to handle this for structs, exceptions, etc as well. We could generate [[deprecated]] on them all the same.

Whereas in C# and Java, we correctly handle this for all kinds of stuff, operations, enums, structs, everything you'd expect.