feat: implement ToSpanTrait for Span<T> to return self

[bengineer42]

Gave Span the ToSpanTrait to help with generics eg:

    fn do_something_on_many<TS, T, +ToSpanTrait<TS, T>>(collection: TS){
        collection.to_span().into_iter().map(|c| c.do_something())
   }

Where do_something_on_many can take Span<T>, Array<T> or [T; N] and in some cases of [T; N] It can solve the Non zero complier issues

[reviewable-StarkWare]

This change is 

[orizi]

Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @bengineer42)

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-- commits line 2 at r1:
add an issue to track what you are actually trying to fix.

[bengineer42]

#9063 linked