Implement unconvert

[dominikh]

Implement https://github.com/mdempsky/unconvert as a simple check.

The main reason we haven't done this so far is that unconvert depends on checking all relevant build configurations, as otherwise it is prone to false positives. We don't have a mechanism for that yet (leading to our own false positives in unused), but we're planning to add a build_targets option, which lets users specify all relevant build targets that staticcheck will check.