automate tuple creation

[Steven Schveighoffer]

On 1/20/22 5:07 PM, forkit wrote:
> On Thursday, 20 January 2022 at 21:16:46 UTC, forkit wrote:
>>
> 
> Cannot work out why I cannot pass valuesArray in as ref const??
> 
> get error: Error: cannot append type `const(int[])[const(int)]` to type 
> `int[][int][]`

Because it would allow altering const data.

e.g.:

```d
const(int[])[const(int)] v = [1: [1, 2, 3]];
int[][int][] arr = [v]; // assume this works
arr[0][1][0] = 5; // oops, just set v[1][0]
```

General rule of thumb is that you can convert the HEAD of a structure to 
mutable from const, but not the TAIL (the stuff it points at).

An associative array is a pointer-to-implementation construct, so it's a 
reference.

-Steve