Improvements to switch

[Nick Treleaven]

On Tuesday, 16 April 2024 at 18:25:45 UTC, Meta wrote:
>     case string: "" -> writeln("empty string"); // Naming the 
> value is optional

Presumably we could leave off the type:
```d
     case : "" -> writeln("empty string");
```
Though we might want to require an identifier or `_` when there 
is a type.

So we'd have this grammar:
```
Case:
     case Pattern -> Statement
     case Type? Identifier Pattern? -> Statement
Pattern:
     `:` AssignExpression
     `if` `(` AssignExpression `)`
```

> int[3] staticArr = [1, 2, 3];
> switch (staticArr) {
>    case int[N], size_t N -> writeln("Static int array of size 
> ", N);

Could have type inference too, like for template parameter 
inference.
```d
     case T[N] _, T, alias N -> writeln("Static ", T.stringof, " 
array of size ", N);
```
Inferred symbols would be listed after matching values.

>     case string[] s: ["D", rest...] -> writeln(rest);     // 
> Prints ["rocks", "!"]

So `rest` would be typed `string[]`.

>     case string[] s: ["D", mid..., "!"]  -> writeln(mid); // 
> Prints "rocks"

I think `mid` cannot be variadic in order for it to be typed 
`string`. So it would be:
```d
     case string[] s: ["D", mid, "!"]  -> writeln(mid); // Prints 
"rocks"
```