Refactoring bindings

[maciejhirsz]

This PR abstracts out bindings in a way that makes iOS and Android extern functions DRY. Type conversion and error handling (throwing exceptions in Java) is done automagically via the export! macro. This also removes a whole bunch of unwrap and expects smell.

It also adds native function for generating QR codes in Rust, fixes #219.

• Introduce error handling for iOS.
• Update C header file.
• Rework string deallocating in Swift bridge.
• Test that iOS compiles and works (it won't until all of the above are done).
• Make Android use KeyPair pointers similar to how iOS was done before.

[pmespresso]

@maciejhirsz is this ready for testing on iOS?

[maciejhirsz]

@yjkimjunior yes please! :)

[pmespresso]

@maciejhirsz the build is failing for me because signer uses an older version of Swift than Xcode10.2 supports. Should this be updated, or should I just go downgrade my Xcode?

[maciejhirsz]

If we can upgrade it I think we should. I think most (all?) of our swift code is internally using just objective C, so there shouldn't be any breakage, but in case there is feel free to ping me on Riot.

[pmespresso]

compiles and no more memory allocation errors, able to create and restore accounts, however, I am able to delete it after i type in just 4 out of 6 of my pin digits. http://g.recordit.co/C7maeWWAWi.gif

[Tbaut]

Also I see on your recording that the qr code doesn't load and remains white, right?

[pmespresso]

Ah yes, that too.

[maciejhirsz]

So everything compiles at last, and the QR code is showing. Need to figure out what's up with account deletion though.

[pmespresso]

leaving this here reference http://g.recordit.co/co1sYl9LrF.gif

[Tbaut]

I've touched this logic last here: https://github.com/paritytech/parity-signer/pull/195/files#diff-ed7c745a0e11d1facbcbeca7e4a54c0aL46 and it might not have been tested enough on iphone

[maciejhirsz]

JS looks fine, just a bit convoluted.

It might be possible that errors aren't propagated properly for iOS on this branch, which is what we use to check if the pin is correct. I'll dig deeper tomorrow to see if I can either figure this out or at least mock something to confirm or reject the hypothesis I have now.

[pmespresso]

@maciejhirsz works well for me!