feat: SpanSerde

[milancermak]

There's no Serde implementation for Spans. That means we can't for example emit events with Spans. This won't compile:

#[contract]
mod Foo {
    use array::SpanTrait;

    #[event]
    fn Sup(values: Span<u8>) {}

    #[external]
    fn do_something(s: Span<u8>) {
        Sup(s);
    }
}

The error is:

error: Trait has no implementation in context: core::serde::Serde::<core::array::Span::<core::integer::u8>>
 --> contract:21:35
        serde::Serde::<Span<u8>>::serialize(@values, ref __data);
                                  ^*******^

error: Trait has no implementation in context: core::serde::Serde::<core::array::Span::<core::integer::u8>>
 --> contract:53:43
                serde::Serde::<Span<u8>>::deserialize(ref data).expect('Input too short for arguments');
                                          ^*********^

Error: Compilation failed.

This PR adds a SpanSerde that fixes the above error. I hope it's just as simple as I imagine it to be, please tell me if I'm missing something.

---

This change is 

[milancermak]

Bump.

Could this still get merged into 1.1.0? I don't care if it's my PR or you do it internally, but people run into this issue all the time, so the sooner it's fixed the better.

[orizi]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @milancermak)

[spapinistarkware]

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @milancermak)

[spapinistarkware]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @milancermak)

[spapinistarkware]

Approved. Just fix the conflicts

Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @milancermak)

[milancermak]

Conflicts resolved. Moved the impl to array.cairo and slightly improved deserialize.

Reviewable status: 0 of 2 files reviewed, all discussions resolved (waiting on @orizi and @spapinistarkware)

[spapinistarkware]

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @milancermak)