Funding an `Account`

[YaronWittenstein]

One of the fields each Transaction should have is the amount field. (it's part of the Envelope).
It could have been part of the Message, but I think that's the right decision to keep it in the Envelope.
(the same holds for the Transaction Nonce field).

The amount records the number of coins to transfer from the Principal account to the Other Account.
The interpretation of the Other Account depends on the Transaction type.

For Spawn Transaction - the Other Account will be the spawned account.
For Call Transaction - the Other Account will be the target account specified in the Transaction Message.

Notes

• For Deploy Transaction, this field seems to be irrelevant. (probably better to keep it as zero or ignore that value).
• For Self-Spawn Transaction this field could also be probably ignored (since the Principal is the spawned Account).