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`.

It's simpler to just use `Self()` and `.0`. Here's an example of how
that changes the output for one method:
```
         pub const fn union(self, other: Self) -> Self {
-            Self::from_bits_retain(self.bits() | other.bits())
+            Self(self.0 | other.0)
         }
```
As well as being simpler, this change makes the code a tiny bit faster
to compile, and results in much better code quality (no function calls)
in dev builds.

Streamline generated code

Prepare for 2.9.3 release