Visibility attributes for function and template parameters

[Quirin Schroll]

Let function and template parameters carry optionally a 
visibility attribute. For example:
```d
void log(
     string message,
     private int line = __LINE__,
     private string func = __FILE_FULL_PATH__
);
```

In another module, `log` can be called, but the `line` parameter 
cannot be set explicitly as it’s not visible. That way, the 
author or a library can ensure that magic default values for some 
parameters are indeed applied. The user has the advantage that it 
is harder to use such a function incorrectly, e.g. `log("current 
step: ", i)` accidentally passes something entirely unrelated to 
`line`.

Another example would be to alleviate the need for a `private` 
overload that has more parameters and is called by the `public` 
one.