Stop using the enum as a manifest constant.

[Mathias LANG]

On Sunday, 6 June 2021 at 10:40:13 UTC, Jack Applegame wrote:
>
> That's not enough. Let's deprecate any use of enum as a 
> manifest constant.

I wouldn't go that far. `enum` are like `#define`, and there 
*are* cases where you want them. E.g. when using them as define 
arguments:
```D
void foo (char[] fmt = DEFAULT_VALUE) { /* ... */ }
```

If `DEFAULT_VALUE` is `immutable`, this will never compile, but 
with `enum`, it might. You'd think that's an odd use case, but we 
actually had a few instances at Sociomantic which triggered when 
we tried to change some internal tools to use `static immutable` 
instead of `enum`.

I agree with the wider point though. People expect manifest 
constant to have an address and to be eagerly evaluated. That's a 
[`static`] `immutable`. `enum` is just a copy-pasted literal.