More Intuitive Syntax for Manifest (enum) Constants

[Per Nordlöw]

As D already has the keyword `macro` reserved, it just struck me 
that I personally think

```d
macro N = 42;
```

is more intuitive and recognizable than

```d
macro N = 42;
```

. Especially for new developers.

This makes it more obvious that D's manifest constants is 
instantly inlined by the compiler opposite to how

`static immutable N = 42;`

is treated.

What do you think?