Use std duration with NaiveTime

[esheppa]

• Add overflowing_add_opt and overflowing_sub_opt, similar to the existing overflowing_add_signed/overflowing_sub_signed. The implementations are a bit more complex as as they can no longer use negation. There may be opportunity for more shared code between overflowing_add_opt and overflowing_sub_opt
• Add abs_diff and duration_since which are similar to signed_duration_since. Implementation of abs_diff is more verbose as it explicitly states all possible cases
• Add and Sub for NaiveTime.
• offset/fixed.rs has some impls updated to use the new APIs
• Change NaiveDateTime impls to use the new API internally (they are now more verbose as have to handle positive and negative cases. These functions will be deprecated in 0.5 anyway (and could be in a future PR which implements similar functions using core::time::Duration)