[Language construct idea] The for loop with prepare step

[Quirin Schroll]

Extend `for` to `for (init; prepare; condition; increment) body`.

The step `prepare` is executed before each check. Ignoring scope, 
the above is lowered to:
```d
     init;
start:
     prepare;
     if (!condition) goto end;
     body;
     increment;
     goto start;
end:
```
`continue` is also `goto start`.

Sometimes, you want to execute something before each condition 
check, no matter if it’s when entering the loop or when looping 
back.

D’s best options for that is:
* Repeat yourself in `init` and `increment`, possibly using a 
local function if the step is more than 1 simple instruction.
* Use `{ prepare; return condition; }()`. While not a DRY 
violation, this has the disadvantage that you cannot declare a 
variable in `prepare` that’s present in the loop’s `increment` or 
`body`.