SortedVectorSet::is_subset is reversed

[zkonge]

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:

pub fn is_subset(&self, other: &SortedVectorSet<T>) -> 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

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

pub fn is_subset(&self, other: &SortedVectorSet<T>) -> bool {
    self.difference(other).next().is_none()
}

This matches the doc comment and standard set semantics.