Improvements to switch

[Nick Treleaven]

On Tuesday, 16 April 2024 at 18:25:45 UTC, Meta wrote:
> - branch guards
> - pattern match on arrays, slices:

Some other things, based on section 3.3 of this C++ proposal:
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2392r2.pdf

- Multiple alternatives that have the same result: ||

E.g.
```d
case :9 || :15 -> "not prime";
```

- Multiple constraints required for a result: &&

E.g.
```d
switch (variant) {
     case int _ && :42 -> "int and 42";
```

- Grouping common names and constraints: { }

E.g.
```d
switch (variant) {
     case int i {
         case if (i < 0) -> "negative int";
         default -> "some other int";
     }
```