Fix kotlin unsigned type conversion

[emarteca]

The type conversion on the FFI boundary was broken in Kotlin for unsigned types -- regular methods and trait methods need to be treated differently. This PR fixes the issue.

[Manishearth]

Seems fine, but why is supports_unsigned necessary? Would love some comments explaining that situation, and then feel free to merge

[emarteca]

Ah, yes, I'll add a comment about this, good point!
The reason is that for non-trait methods (on opaques/structs/enums), the methods are being called via JNA, which is built in Java, and Java doesn't support unsigned types. Because of this, we need the defined JNA Library to only use signed types in the method signatures, and convert from unsigned to signed and back as required so that users of the methods can pass in/receive the unsigned types that match the API.

But, for the generated traits, we don't have a JNA Library -- the trait methods are meant to be implemented by the user, and so they should have the expected (unsigned) API. The methods are called directly from Rust, so the JNA lack of support for unsigned types doesn't apply.

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That being said, the actual right way to do this is to subclass the JNA IntegerType and make a representation for all the unsigned types, since that's allowed over the FFI layer. I have someone blocked on this fix right now though, so I'm thinking I'll push this solution and then fix the issue properly in the next couple days when it's less urgent: #748

[Manishearth]

Makes sense, land once you have the code commented

[emarteca]

Ok sweet, thanks!