execution-core: add serde feature

[Neotamandua]

Summary

Introduce a serde feature to execution-core that allows serde Serialize & Deserialize to be used on structs that are also used on the integration side (for example, all event structs from the genesis contracts).

This issue relies on: https://github.com/dusk-network/bls12_381-bls/issues/21

[d-sonuga]

The types used by event structs, along with relevant subfields defined in other dusk crates, are

• ContractId (piecrust_uplink)
• StealthAddress (phoenix_core), needed in WithdrawReceiver for ConvertEvent and WithdrawEvent
  • JubJubExtended (alias for ExtendedPoint in dusk-jubjub) - Fq (alias for Scalar in dusk-bls12_381)
  • NotePublicKey (PublicKey in jubjub-schnorr) - JubJubExtended
• Scalar in dusk-bls12_381 is also needed in PhoenixTransactionEvent
• Note (phoenix_core), needed in PhoenixTransactionEvent
  • JubJubAffine (alias for AffinePoint in dusk-jubjub) - Fq (alias for Scalar in dusk-bls12_381)
  • Sender - JubJubAffine
• PublicKey (bls12_381-bls) used in SlashEvent, ConvertEvent, DepositEvent, ContractToAccountEvent, StakeKeys which is used in StakeEvent and WithdrawReceiver which is used in WithdrawEvent.

Following from this, serde Serialize and Deserialize implementations will be needed in:

• Scalar in dusk-bls12_381
• Note, Sender and StealthAddress in phoenix-core
• AffinePoint and ExtendedPoint in dusk-jubjub
• PublicKey in bls12_381-bls
• PublicKey in jubjub-schnorr
• ContractId in piecrust-uplink