Variable-sized Coin serialization

[sisou]

What's in this pull request?

This PR makes Coin serialize to a varint in transactions and accounts, both over the wire and their internal representations (history store).

Reasoning

No transactions will ever make use of the full u64 range for value, and especially not for fee. So most zero bytes for Coins in most transactions are wasted. This is especially true for signaling transactions, which have 16 zero-bytes to represent 0 value and (mostly) 0 fee. This PR reduces them to two zero bytes.

Exceptions

There are two exceptions for backwards-compatibility with PoW transactions:

1. Coins in vesting contract creation data are still serialized with fixint::be
2. For determining a transaction's hash, tx.serialize_content() serializes value and fee as before with fixint::be

Pull request checklist

• All tests pass. The project builds and runs.
• I have resolved any merge conflicts.
• I have resolved all clippy and rustfmt warnings.

[hrxi]

Looks like a good improvement to me.