Simple refactoring, renaming things to be clearer

[nczhu]

Renaming the following:

UnspentOutputs -> UtxoStore: map of hash and transactionOutputs

TransactionInput (s):

• outpoint : UTXO hash corresponding to tOutputs in utxoStore
• sigScript : Signature of current Transaction, by the corresponding key as the outpoint; The script contains two components, a signature and a public key. The public key must match the hash given in the script of the redeemed output. The public key is used to verify the redeemers signature, which is the second component. More precisely, the second component is an ECDSA signature over a hash of a simplified version of the transaction.

TransactionOutput (s):

• value : no change
• pubKeyScript : the publickey condition in order to spend this utxo

Type names:

• Un-alias confusing typenames, just use the H512, H256 original naming
• leftover -> reward
• initial_utxo -> genesis_utxo