Un-parameterize process output

So that the return type of `execute` and `executeShell` are compatible.

Long story short, because the structure was declared in the template, it created two different types. This prevented the use of a single `ret` object:
```D
auto ret = execute(...); //First declaration
ret = execute(...); //Ok
ret = executeShell(...); //Nope
```
So this "unlocks" that.

@schveiguy , I think you can review this?