fit-promise library

This was formerly part of the lib/fit library. Further migrations and renamings are expected.

fit::promise, fit::future, fit::executor, etc.

• When writing asynchronous event-driven programs, it's convenient to be able to stage a sequence of asynchronous tasks. This tends to be challenging to implement in a callback-driven manner due to object lifetime issues, so we would like an alternative pattern that is easier to apply correctly, such as by expressing asynchronous logic as a compositions of futures.
• The C++ 14 standard library offers std::future but it is tied to a thread-based execution model. Awaiting a future requires blocking, which is bad for event loops.
• So libfit offers a family of APIs that work better with event loops.