The generated code for various operations has local variables:
```
        pub const fn union(self, other: Self) -> Self {
            let f = self;
            let other = other;
            { Self::from_bits_retain(f.bits() | other.bits()) }
        }
```
These are present due to the way the relevant macros are structured.
This commit restructures the `__impl_bitflags` macro to avoid these
indirections, resulting in this code instead:
```
        pub const fn union(self, other: Self) -> Self {
            Self(self.0 | other.0)
        }
```
Benefits:
- `cargo expand` output is nicer.
- It is a little faster to compile.
- `__impl_bitflags!` becomes simpler, with many fewer parameters. E.g.
  `$self` instead of `$union0`, `$intersect0`, etc.
- The `__impl_bitflags!` call sites are also a little nicer, using
  `&self` instead of `f`.