DoubleEndedIterator implementation for Segments

[rushiiMachine]

Existing Functionality

Currently there is only a forward iterator for Segments even though it is backed by a known size slice.

https://github.com/SergioBenitez/Rocket/blob/f1f533c1e5b0df5b44877d7cca39fb0f596a21b6/core/http/src/uri/segments.rs#L279-L295

Suggested Changes

Add a DoubleEndedIterator implementation to Segments to allow for iterating path segments in reverse for example, if be necessary.

Alternatives Considered

Collecting into a Vec and iterating in reverse is inefficient and for-i is just ugly and doesn't give clean flexibility.

[SergioBenitez]

I'm not opposed to this, but the implementation would require great care. The inherent methods on Segments all assume that the iterator only moves forward, so something would need to be done about this.

[GentBinaku]

Can I work on it?

[SergioBenitez]

Yes, please feel free!