I feel that this is a very vital, missing piece of functionality. This adds on to #13072.
Only bits used in the definition of the bitflag are considered for the universe set. This is a bit safer than simply inverting all of the bits in the wrapped value.
```rust
bitflags!(flags Flags: u32 {
FlagA = 0x00000001,
FlagB = 0x00000010,
FlagC = 0x00000100,
FlagABC = FlagA.bits
| FlagB.bits
| FlagC.bits
})
...
// `Not` implements set complement
assert!(!(FlagB | FlagC) == FlagA);
// `all` and `is_all` are the inverses of `empty` and `is_empty`
assert!(Flags::all() - FlagA == !FlagA);
assert!(FlagABC.is_all());
```
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