Syntax ideas for `ref` returning delegate and FP parameters

[Boris Carvajal]

On Saturday, 10 April 2021 at 14:27:31 UTC, Q. Schroll wrote:
> Do you guys have any better ideas? I don't really like mine.

https://github.com/dlang/dmd/pull/10018#issuecomment-504347888

In short, allow putting function/delegate keyword at the 
beginning like function literals do:

```d
// declaration
function int(char c) fp = function int(char c) { ... };

// also function literal syntax already accepts 'ref' returns
function ref int(char c) fp = function ref int(char c) { ... };

// as parameter
void fun(ref function ref int(char c) fp);
```