Absence of isAllocator trait

[Per Nordlöw]

On Monday, 6 September 2021 at 15:46:52 UTC, Paul Backus wrote:
> * `void allocate(size_t size)`

Should be

* `void[] allocate(size_t size)`

Thanks. Here's what I have so far

```d
enum isAllocator(T) = (is(typeof(T.allocate(size_t.init)) == 
void[]) &&
                        is(typeof(T.alignment) == uint));
```