Note object refactoring

[bobbinth]

This is a subset to #464 that is limited to the Note and a few related objects. One of the objects of this is to provide an "intuitive" way to think about note objects but also to separate core note contents from note metadata. The objects involved in this refactoring are Note and NoteEnvelope which currently look like so:

pub struct Note {
    assets: NoteAssets,
    metadata: NoteMetadata,
    recipient: NoteRecipient,

    id: NoteId,
    nullifier: Nullifier,
}

pub struct NoteEnvelope {
    note_id: NoteId,
    note_metadata: NoteMetadata,
}

I am thinking of refactoring of renaming NoteEnvelope into NoteHeader and refactoring these as follows:

pub struct NoteHeader {
    id: NoteId,
    metadata: NoteMetadata,
}

pub struct NoteDetails {
    assets: NoteAssets,
    recipient: NoteRecipient,
}

pub struct Note {
    header: NoteHeader,
    details: NoteDetails,

    nullifier: Nullifier,
}

This has a couple of benefits:

1. It roughly follows the structure the Block struct of having the objects consist of header and contents.
2. I think NoteHeader is a bit more intuitive name as compared to NoteEnvelope.
3. It more closely matches how things work on the node and the client (e.g., sometimes we manipulate note details separately from note metadata).
4. It also should match more closely some of the protobuf messages we use in the node.

Once implemented, this should unblock #412.

@hackaugusto, @polydez, @igamigo - would appreciate any feedback on this.

[hackaugusto]

related: #350

[hackaugusto]

I left my comment on #350, but basically the idea would be to have a structure for each hash operation.

[bobbinth]

Closed by #664.