Rename Transaction::hash()

[conradoplg]

Motivation

With ZIP-244, the transaction ID is no longer simply the hash of the transaction bytes (though it's still a hash...)
For this reason, it could be better to rename the hash() method to id() or txid() to make this clearer.

It may also be nice to:

• Rename transaction::Hash too.
• Refactor txid.rs. Should it be incorporated into Transaction, since it's relatively small? See discussion

Specifications

N/A

Designs

N/A

Related Work

Follow up to #2129

[conradoplg]

I still would like to this but it's not important nor required.