refactor(AztecNr): Unify isEmpty/isZero and empty/zero API and Empty trait

[benesjan]

We don't have consistent naming of functions which return an instance of an "empty"/"zero" object and the related functions (isEmpty etc.).

We diverged in a few places from empty because it became unclear. One of these cases was when we I changed the naming of AppendOnlyTreeSnapshot::empty to ::zero because there it was not clear whether it's a snapshot of a tree where all the nodes are zero or whether it represents a tree where the leaves are 0 but all the other levels are hashed.

I would use "zero" in cases where all the values are set to 0/are "falsey" and empty in the rest of the cases.

[sklppy88]

@benesjan:
A couple questions here.

1. I see a trait for Empty, but not for Zero.
   Can / should I add a Zero trait?
2. Why do some structures implement both an empty() and a zero() which return the exact same thing, I would like to prune this to only implement one at a time. Is it okay to nuke one of them, or is there a specific reason for having both ? (see EthAddress)

A heuristic I'm planning to use to tackle this is as follows:

If it is a data structure that holds primitives only -> is_zero
If it is a data structure that holds at least one non-primitives -> is_empty

Some examples of applications of the rule (current -> after):
NoteHeader (empty -> empty; same)
ReadRequestStatus (empty -> zero)
NullifierLeafPreimage (empty -> zero)
NullifierKeyValidationRequestContext (empty -> empty; same)
CallerContext (empty -> empty; same)
NullifierKeyValidationRequest (empty -> empty; same)
SideEffect (empty -> zero)
ReadRequest (empty -> zero)
SideEffectLinkedToNoteHash (empty -> zero)
L2ToL1Message (empty -> empty)
etc

This seems to make sense as a general rule, esp in the case of NullifierLeafPreimage, SideEffect etc as an "empty" side effect is not really clear, a zero one is a bit more accurate imo.

If anything it could make more sense to even move everything to zero just for consistency, even the things that currently do make sense as empty (the "containers" such as NoteHeader, CallerContext, etc). As zero is still generally accurate even for them.

wdyt ?