Set operations

#2 and #3 instead?
- `lset<=`
- `lset=`
- `lset-adjoin`
- `lset-union`
- `lset-intersection`
- `lset-difference`
- `lset-xor`
- `lset-diff+intersection`
- `lset-union!`
- `lset-intersection!`
- `lset-difference!`
- `lset-xor!`
- `lset-diff+intersection!`