Bump to v0.21.0

[moCello]

0.21.0 - 2021-10-14

Added

• Add asyncifying FFI imports
• Add unstake function to allow unstaking a previously staked amount [Upgrade staking functionality to new specs #58]
• Add fetch_existing_nullifiers to the StateClient [Wallet assumes all notes are unspent #41]
• Add ff dependency at 0.13

Changed

• Change FFI to take pointers to u64
• Change withdraw function to withdraw the reward for staking and
  participating in the consensus [Upgrade staking functionality to new specs #58]
• Change stake and withdraw cryptographic signatures to what the stake
  contract expects in the new spec [Upgrade staking functionality to new specs #58]
• Change StakeInfo to have an optional amount staked, a reward, and a
  signature counter [Upgrade staking functionality to new specs #58]
• Change signature of fetch_notes by removing block height
• Change fetch_notes in the FFI to delegate buffer allocation to the user. [Upgrade staking functionality to new specs #58]
• Change note picker to pick the largest amount of notes possible, with the minimum
  total value [Change note picking to pick the maximum number of notes #55]
• Change get_balance to return total and max spendable [Return amount spendable in get_balance #53]
• Change fetch_existing_nullifiers in the FFI to return early [Panic emitted during execution of the WASM module #49]
• Modify StateClient to receive full stake info [Send the correct arguments for stake contract calls #46]
• Change Canon encoding length of Transaction [Transaction needs to serialize as transfer-contract::Call::Execute #31]
• Change transacting functions to return Transaction [Return TX Hash (or full TX) on wallet methods #40]
• Update transaction to generate change output
• Change transactions to have an optional crossover
• Update STCT to use Schnorr signatures [Replace bls signature for schnorr signature in STCT #34]
• Update dependencies
• Update phoenix-core to 0.17
• Update dusk-pki to 0.11
• Update dusk-schnorr to 0.11
• Update dusk-jubjub to 0.12
• Update dusk-poseidon to 0.26
• Update dusk-plonk to 0.12
• Update dusk-bls12_381-sign to 0.4
• Update dependencies
• Update phoenix-core 0.18 -> 0.20.0-rc.0
• Update dusk-pki 0.11 -> 0.12
• Update dusk-schnorr 0.12 -> 0.13
• Update dusk-poseidon 0.29.1-rc.0 -> 0.30
• Update dusk-plonk 0.13 -> 0.14
• Change dusk-merkle dependency to poseidon-merkle after merkle crate separation
• Update phoenix-core 0.20.0-rc.0 -> 0.21
• Update dusk-pki 0.12 -> 0.13
• Update dusk-schnorr 0.13 -> 0.14
• Update dusk-poseidon 0.30 -> 0.31
• Update dusk-plonk 0.14 -> 0.16
• Update dusk-bls12_381-sign 0.4 -> 0.5
• Update dusk-jubjub 0.12 -> 0.13
• Update poseidon-merkle 0.2.1-rc.0 -> 0.3
• Update rusk-abi 0.10.0-piecrust.0.6 -> 0.11

Fixed

• Fix encoded_len in Transaction [Canon implementation incorrect on Transaction #44]
• Fix notes from fetch_notes being assumed unspent [Wallet assumes all notes are unspent #41]
• Fix fee generation in stake and withdrawal

Removed

• Remove get_block_height from the StateClient trait [Upgrade staking functionality to new specs #58]
• Delete extend_stake since the stake contract removed it [Send the correct arguments for stake contract calls #46]