SortedVectorSet::is_subset is reversed

In `sorted_vector_map` v0.2.0, `SortedVectorSet::is_subset` appears to implement the opposite relation.

**Current docs:**
`self` is a subset of `other` (i.e. `self ⊆ other`)

**Current code:**
```rust
pub fn is_subset(&self, other: &SortedVectorSet) -> bool {
other.difference(self).next().is_none()
}
```

`other.difference(self).is_empty()` checks whether `other ⊆ self`, so the implementation is reversed.
`is_superset` delegates to `other.is_subset(self)`, so it becomes reversed as well.

### Repro
```rust
use sorted_vector_map::sorted_vector_set;

#[test]
fn subset_semantics() {
let a = sorted_vector_set! { 1, 2 };
let b = sorted_vector_set! { 1, 2, 3 };

assert!(a.is_subset(&b)); // expected true
assert!(!b.is_subset(&a)); // expected false

assert!(b.is_superset(&a)); // expected true
assert!(!a.is_superset(&b)); // expected false
}
```

### Suggested fix
```rust
pub fn is_subset(&self, other: &SortedVectorSet) -> bool {
self.difference(other).next().is_none()
}
```

This matches the doc comment and standard set semantics.