FutureX 0.14

• Method observe(on:) is more flexible, it can now be used to runs transformations like map, tryMap on a specified queue (and actually any other transformations too, it composes really well with any of them).
• Instead of a convenience Future { succeed, fail in } we now have Future { promise in } which is consistent with the regular way you create Promise/Future pair and also more flexible and performant.
• Inline the first callback in Promise. It's very often when there is only one callback for each Promise. These operations are now up to 15% faster.
• Implement CustomDebugStringConvertible for Promise