Add feature `no_panic`

[malbarbo]

I need this feature for a project.

I've started working on it here.

I would like to known if this feature is interesting to upstream.

The existing panic behavior is from indexing and unwrapping. For the indexing I created a trait GetAt and used it instead of direct indexing. The trait has an implementation that uses [] and other that uses get_unchecked. For the unwrapping, I used if let and panic in the else clause. But there is the question of what to do in non panic mode.

[Manishearth]

I would prefer to use checked indexing where possible, instead, with reasonable fallback behavior and debug assertions.