Merge: Revamp Clock to return float values for easier memory management

This PR updates the main services of `Clock` to return float values instead of malloced extern instances of `Timespec`. There is a precision loss, but it's sufficient for games and the likes, and a program needing higher precision can still use `Timespec` directly. Clients don't even have to worry about memory management anymore.

Pull-Request: #2117
Reviewed-by: Jean Privat <jean@pryen.org>