refactor(deps): use conflate instead of merge crate

[simonsan]

conflate (essentially merge 0.1.0-dev) lacks the blanket impl<T> Merge for Option<T>, which was removed from version 0.1.0

impl<T> Merge for Option<T> {
    fn merge(&mut self, mut other: Self) {
        if !self.is_some() {
            *self = other.take();
        }
    }
}

Hence, I added the overwrite_none strategies where needed.

We could also add it back, for a future version, though I think there was some decision involved, that strategies should be more explicit, because it could be confusing for strategies being applied without having an annotation. So I felt, I don't want to add that back to conflate for now.

Fixes #258

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 45.9%. Comparing base (25661d4) to head (05d79b2).

Additional details and impacted files

Files with missing lines	Coverage Δ
crates/core/src/backend/ignore.rs	47.1% <ø> (+0.7%)	⬆️
crates/core/src/commands/backup.rs	62.5% <ø> (ø)
crates/core/src/commands/forget.rs	85.7% <ø> (ø)
crates/core/src/repofile/snapshotfile.rs	49.7% <ø> (+0.2%)	⬆️
crates/core/src/repository.rs	48.1% <ø> (ø)

... and 18 files with indirect coverage changes

[simonsan]

Removed/reverted the 'use provided hashmap strategy' commit, as that contains a breaking change for rustic and rustic_core (or rather rustic_backend? or both?) users, which we should do in another follow-up PR.

EDIT: Breaking change is in #291

[aawsome]

LGTM