Serde for signed integers

[shramee]

We can have Serde implemented in at least two ways,

1. inner integer and sign bool in separate felts,

2. sign added as 33rd bit on top of inner , putting everything in one felt.

For example,

Value	Serialized	Binary
i33 {inner: 1, sign: false}	1	0b1
i33 {inner: 1, sign: true}	4294967297	0b100000000000000000000000000000001

 PS, `0b100000000000000000000000000000000` and `0b0` both point to `0` one with a `sign` and other without.

Example Implementation

impl I33Serde of Serde<i33> {
    fn serialize(self: @i33, ref output: Array<felt252>) {
        let mut val_felt: felt252 = (*self.inner).into();
        if *self.sign {
            val_felt = val_felt + 4294967296; // 2 ^ 32 = 4294967296
        }
        val_felt.serialize(ref output);
    }
    fn deserialize(ref serialized: Span<felt252>) -> Option<i33> {
        let val_felt: felt252 = Felt252Serde::deserialize(ref serialized)?;
        let val_u32_option: Option<u32> = val_felt.try_into();
        Option::Some(
            if val_u32_option.is_some() {
                // felt value fits u32, sign is false
                i33 { inner: val_u32_option?, sign: false,  }
            } else {
                // felt value exceeds u32, sign set on 33rd bit
                let inner: u32 = (val_felt - 4294967296).try_into()?;
                i33 { inner, sign: true,  }
            }
        )
    }
}

To me, the second way seems better, more semantic. Let me know what we think.