Bind the intention for there to be a 1:1 wallet<-->sync-state relation at the type level

[zancas]

For example, this function takes and sync state and some scan ranges and binds them without any explicit connection to the (assumed) containing wallet:

zingolib/pepper-sync/src/sync.rs

Line 850 in 3789344

pub fn add_scan_targets(sync_state: &mut SyncState, scan_targets: &[ScanTarget]) {

I've experimented with this a little. I am not sure about the best mutability pattern(s) to use.

I also notice that the "bool" return of .insert is silently ignored and would like to upgrade the code to use that event.