Expanding CTFE code during compilation

[Azi Hassan]

Hello,

Apologies in advance if this has been asked before. I can't find 
the right words to express what I'm looking for, but essentially, 
I'm wondering if the offers has the option of executing the parts 
that can be evaluated at compile time and then replacing them 
with the result of this evaluation. The C equivalent would be 
compiling with the -E flag as explained here : 
https://stackoverflow.com/questions/4900870/can-gcc-output-c-code-after-preprocessing

For example :

```
//main.d
import std;

void main()
{
     enum x = iota(1, 5).reduce!"a * b";
     x.writeln();
}
```

Then after running something like

```
dmd --expand-ctfe main.d -o expanded.d
cat expanded.d
```

We'd get :

```
//expanded.d
import std;

void main()
{
     24.writeln();
}
```

Just in case this is a consequence of the XY problem, the reason 
why I'm looking for this is to make sure that the code I wrote 
did evaluate to what I'm expecting it to. Right now I do this 
with an enum assignment followed by static asserts, but I'd love 
it for there to be some sort of visual feedback.

Thanks in advance